What is the meaning of the word 'Emptiness' as it was used in the oldest texts?

A lengthy comment on Ruben's answer;

I think Thanissaro wasn't necessarily wrong in meaning but is rather sloppy there saying;

"This process is pursued until it arrives at the "themeless concentration of awareness." When noting that even this refined level of concentration is fabricated, inconstant, and subject to cessation, one gains total release from all mental fermentations and the disturbances that would arise based on them.

The meaning should be close to this for it to be an agreeable expression to me;

The release is conditioned in as far as there is a conditioning of aggregates leading to the release but the truth & reality of the release itself is unconditioned.

A person does not become unsatisfied with the release but he becomes disenchanted with the aggregates that are grasped with wrong view as personal and are that from which there is a release.

It is the prior development of the aggregates that conditions emergence from the sannavedananirodha and these are only fully exhausted at parinibbana.

"What is the difference between one who is dead, who has completed his time, and a monk who has attained the cessation of perception & feeling?"

"In the case of the one who is dead, who has completed his time, his bodily fabrications have ceased & subsided, his verbal fabrications ... his mental fabrications have ceased & subsided, his vitality is exhausted, his heat subsided, & his faculties are scattered. But in the case of a monk who has attained the cessation of perception & feeling, his bodily fabrications have ceased & subsided, his verbal fabrications ... his mental fabrications have ceased & subsided, his vitality is not exhausted, his heat has not subsided, & his faculties are exceptionally clear. This is the difference between one who is dead, who has completed his time, and a monk who has attained the cessation of perception & feeling." https://www.accesstoinsight.org/tipitaka/mn/mn.043.than.html

How the Blessed One Passed into Nibbana

9. And the Blessed One entered the first jhana. Rising from the first jhana, he entered the second jhana. Rising from the second jhana, he entered the third jhana. Rising from the third jhana, he entered the fourth jhana. And rising out of the fourth jhana, he entered the sphere of infinite space. Rising from the attainment of the sphere of infinite space, he entered the sphere of infinite consciousness. Rising from the attainment of the sphere of infinite consciousness, he entered the sphere of nothingness. Rising from the attainment of the sphere of nothingness, he entered the sphere of neither-perception-nor-non-perception. And rising out of the attainment of the sphere of neither-perception-nor-non-perception, he attained to the cessation of perception and feeling.

10. And the Venerable Ananda spoke to the Venerable Anuruddha, saying: "Venerable Anuruddha, the Blessed One has passed away."

"No, friend Ananda, the Blessed One has not passed away. He has entered the state of the cessation of perception and feeling."[59]

11. Then the Blessed One, rising from the cessation of perception and feeling, entered the sphere of neither-perception-nor-non-perception. Rising from the attainment of the sphere of neither-perception-nor-non-perception, he entered the sphere of nothingness. Rising from the attainment of the sphere of nothingness, he entered the sphere of infinite consciousness. Rising from the attainment of the sphere of infinite consciousness, he entered the sphere of infinite space. Rising from the attainment of the sphere of infinite space, he entered the fourth jhana. Rising from the fourth jhana, he entered the third jhana. Rising from the third jhana, he entered the second jhana. Rising from the second jhana, he entered the first jhana.

Rising from the first jhana, he entered the second jhana. Rising from the second jhana, he entered the third jhana. Rising from the third jhana, he entered the fourth jhana. And, rising from the fourth jhana, the Blessed One immediately passed away. https://www.accesstoinsight.org/tipitaka/dn/dn.16.1-6.vaji.html

Until Parinibbana the yogi will be said to be entering into & emerging from the percepience of extinguishment, whereas at parinibbana the faculties are scattered and life-force extinguished, therefore there is nothing left to be grasped as a person and one therefore does not say that a person is abiding in parinibbana or is going to be emerging from it.

In regards to the following;

"Very good, venerable sir." And, delighting in and approving of Ven. Kamabhu's answer, Citta asked him a further question: "When a monk has emerged from the cessation of perception & feeling, how many contacts make contact?"

"When a monk has emerged from the cessation of perception & feeling, three contacts make contact: contact with emptiness, contact with the signless, & contact with the undirected."[3]

Theravadin comy says;

Emptiness, the signless, & the undirected are names for a state of concentration that lies on the threshold of Unbinding. They differ only in how they are approached. According to the commentary, they color one's first apprehension of Unbinding: a meditator who has been focusing on the theme of inconstancy will first apprehend Unbinding as signless; one who has been focusing on the theme of stress will first apprehend it as undirected; one who has been focusing on the theme of not-self will first apprehend it as emptiness

I think it is appropriate to say that signless release removes taints in regards to this excerpt

There are, monks, three unskilled ways of thought: thoughts of lust, thoughts of ill-will, thoughts of hurting. And these three unskilled states disappear utterly in him whose heart is well established in the four foundations of mindfulness, or who practices concentration on the signless. https://www.accesstoinsight.org/tipitaka/sn/sn22/sn22.080.wlsh.html

In that sense it is on the treshold of nibbana

“Venerable sir, it is said, ‘the removal of lust, the removal of hatred, the removal of delusion.’ Of what now, venerable sir, is this the designation?”

“This, bhikkhu, is a designation for the element of Nibbāna: the removal of lust, the removal of hatred, the removal of delusion. The destruction of the taints is spoken of in that way.”

When this was said, that bhikkhu said to the Blessed One: “Venerable sir, it is said, ‘the Deathless, the Deathless.’ What now, venerable sir, is the Deathless? What is the path leading to the Deathless?”

“The destruction of lust, the destruction of hatred, the destruction of delusion: this is called the Deathless. This Noble Eightfold Path is the path leading to the Deathless; that is, right view … right concentration.” https://suttacentral.net/sn45.7/en/bodhi

Nibbana and other words are sometimes spoken of in a qualified and sometimes in a definitive sense;

“Reverend, they speak of a person called ‘personal witness’. What is the personal witness that the Buddha spoke of?”

“First, take a mendicant who, quite secluded from sensual pleasures … enters and remains in the first absorption. They meditate directly experiencing that dimension in every way. To this extent the Buddha spoke of the personal witness in a qualified sense.

Furthermore, take a mendicant who, as the placing of the mind and keeping it connected are stilled, enters and remains in the second absorption … third absorption … fourth absorption. They meditate directly experiencing that dimension in every way. To this extent the Buddha spoke of the personal witness in a qualified sense.

Furthermore, take a mendicant who, going totally beyond perceptions of form, with the ending of perceptions of impingement, not focusing on perceptions of diversity, aware that ‘space is infinite’, enters and remains in the dimension of infinite space. They meditate directly experiencing that dimension in every way. To this extent the Buddha spoke of the personal witness in a qualified sense. Furthermore, take a mendicant who enters and remains in the dimension of infinite consciousness … the dimension of nothingness … the dimension of neither perception nor non-perception …

Furthermore, take a mendicant who, going totally beyond the dimension of neither perception nor non-perception, enters and remains in the cessation of perception and feeling. And, having seen with wisdom, their defilements come to an end. They meditate directly experiencing that dimension in every way. To this extent the Buddha spoke of the personal witness in a definitive sense.”

or here;

“First, take a mendicant who, quite secluded from sensual pleasures … enters and remains in the first absorption. To this extent the Buddha spoke of extinguishment in the present life in a qualified sense. …

Furthermore, take a mendicant who, going totally beyond the dimension of neither perception nor non-perception, enters and remains in the cessation of perception and feeling. And, having seen with wisdom, their defilements come to an end. To this extent the Buddha spoke of extinguishment in the present life in a definitive sense.”

Here another relevant part of the mn43 that should explain how it is prior development that is the cause for attainment, a cause for persistence and a cause for the emerging from and not the release itself or something released that upholds the truth of the release;

"Very good, venerable sir." And, delighting in and approving of Ven. Kamabhu's answer, Citta asked him a further question: "Now, how does emergence from the cessation of perception & feeling come about?"

"The thought does not occur to a monk as he is emerging from the cessation of perception & feeling that 'I am about to emerge from the cessation of perception & feeling' or that 'I am emerging from the cessation of perception & feeling' or that 'I have emerged from the cessation of perception & feeling.' Instead, the way his mind has previously been developed leads him to that state."

"Very good, venerable sir." And, delighting in and approving of Ven. Kamabhu's answer, Citta asked him a further question: "When a monk is emerging from the cessation of perception & feeling, which things arise first: bodily fabrications, verbal fabrications, or mental fabrications?"

"When a monk is emerging from the cessation of perception & feeling, mental fabrications arise first, then bodily fabrications, then verbal fabrications."

"Very good, venerable sir." And, delighting in and approving of Ven. Kamabhu's answer, Citta asked him a further question: "When a monk has emerged from the cessation of perception & feeling, how many contacts make contact?"

"When a monk has emerged from the cessation of perception & feeling, three contacts make contact: contact with emptiness, contact with the signless, & contact with the undirected." https://www.accesstoinsight.org/tipitaka/sn/sn41/sn41.006.than.html

And this;

"How many conditions are there for the attainment of the theme-less awareness-release?"

"There are two conditions for the attainment of the theme-less awareness-release: lack of attention to all themes and attention to the theme-less property. These are the two conditions for the attainment of the theme-less awareness-release."

"And how many conditions are there for the persistence of the theme-less awareness-release?"

"There are three conditions for the persistence of the theme-less awareness-release: lack of attention to all themes, attention to the theme-less property, and a prior act of will. These are the three conditions for the persistence of the theme-less awareness-release."

"And how many conditions are there for the emergence from the theme-less awareness-release?"

"There are two conditions for the emergence from the theme-less awareness-release: attention to all themes and lack of attention to the theme-less property. These are the two conditions for the emergence from the theme-less awareness-release." -mn43

The word 'extinguishment-cessation-element' means something close to an end of the burning things there; it doesn't mean that it is something that is literally left in the ashes and is in that sense not to be construed as a thing among the cessant conditioned elements that were burning and it is likewise with the extinguishment-cessation of that which is discerned to be changing as it persists [the conditioned: sankhata].

That state being the Asoka-state [sorrowless; without suffering], which is also an attainment of the deathless; and that seeing with wisdom is said to be a destruction of taints for a person.

This doing eventually makes it so one emerges an arahant, without defilement, this is said to be the foremost release and as a completed destruction of taints

venerable sir, whenever we want, by completely surmounting the base of neither-perception-nor-non-perception, we enter upon and abide in the cessation of perception and feeling. And our taints are destroyed by our seeing with wisdom. Venerable sir, this is another superhuman state, a distinction in knowledge and vision worthy of the noble ones, a comfortable abiding, which we have attained by surmounting the preceding abiding, by making that abiding subside. And, venerable sir, we do not see any other comfortable abiding higher or more sublime than this one.”

I have nothing to describe endlessly?

Transcending form, leaving stress behind, perception shifts to space:

MN121:6.1: Furthermore, a mendicant—ignoring the perception of wilderness and the perception of earth—focuses on the oneness dependent on the perception of the dimension of infinite space.

Transcending space, leaving stress behind, perception shifts to consciousness:

MN121:7.1: Furthermore, a mendicant—ignoring the perception of earth and the perception of the dimension of infinite space—focuses on the oneness dependent on the perception of the dimension of infinite consciousness.

Transcending consciousness, leaving stress behind, perception shifts to nothingness:

MN121:8.1: Furthermore, a mendicant—ignoring the perception of the dimension of infinite space and the perception of the dimension of infinite consciousness—focuses on the oneness dependent on the perception of the dimension of nothingness.

Continuing thus, beyond signs, beyond even the signless, leaving stress behind, perception shifts to emptiness, without defilements:

MN121:12.4: They understand: ‘This field of perception is empty of the perception of the defilements of sensuality, desire to be reborn, and ignorance.
MN121:12.5: There is only this that is not emptiness, namely that associated with the six sense fields dependent on this body and conditioned by life.’
MN121:12.6: And so they regard it as empty of what is not there, but as to what remains they understand that it is present.
MN121:12.7: That’s how emptiness is born in them—genuine, undistorted, and pure.

That is a summary of how emptiness is understood per MN121. Everything else is simply stressful commentary.

This is well explained by Ven. Thanissaro in his commentary to MN 122:

This sutta gives many valuable lessons on practical issues surrounding the attempt to develop an internal meditative dwelling of emptiness, to maintain it, and to see it through to Awakening. Some of these issues include the need for seclusion as a conducive setting for the practice, types of conversation and thinking that are beneficial and harmful for the practice, the dangers of being distracted by visitors, and the proper attitude to have toward one's teacher. However, for an explanation of emptiness in and of itself, it's necessary to look elsewhere in the Canon.

There you find emptiness approached from three perspectives, treating it (1) as a meditative dwelling, (2) as an attribute of objects, and (3) as a type of awareness-release. The first approach is obviously the most immediately relevant to the discussion in this sutta, but in fact all three approaches play a role here.

Emptiness as a meditative dwelling is most fully discussed in MN 121. Essentially, it boils down to the ability to center the mind in a particular mode of perception, to maintain it there, and then to notice the absence and presence of disturbance within that mode. The process starts with perceptions of one's external surroundings — village, wilderness, the earth property — and then moves internally to the four formless states, the "themeless concentration of awareness," and finally to release from all mental fermentation. Each step is compared to the one preceding it to see how its more refined perception engenders less disturbance. For instance, if you move from a perception of the wilderness to a perception of earth, the first step is to settle and "indulge" in that perception. Then you notice what types of disturbance have been abandoned in the move from the perception of wilderness to the perception of earth — for example, all thought of the dangers of wilderness are gone — and then to see what disturbances remain based on the latter perception. Then you abandon the perception causing those disturbances and move on to a more refined level of perception. This process is pursued until it arrives at the "themeless concentration of awareness." When noting that even this refined level of concentration is fabricated, inconstant, and subject to cessation, one gains total release from all mental fermentations and the disturbances that would arise based on them. This is the level of emptiness that is "superior and unsurpassed," and is apparently what the Buddha is referring to in this sutta when he says that by "not attending to any themes, he enters & remains in internal emptiness."

Notice that in every step along the way of this process, the emptiness is the lack of disturbance experienced in a particular mind state. This means that the mind state is to be perceived simply as an example of the presence and absence of stress. In other words, emptiness in this sense relates directly to the second of the three characteristics — stress or suffering. The pursuit of this emptiness relates to the four noble truths, as it looks for the causes of stress and uses tranquility together with insight to abandon those causes in a quest to put a total end to suffering.

Emptiness in its second meaning, as an attribute of objects, is most fully discussed in SN 35.85. That sutta describes emptiness as meaning the lack of self or anything pertaining to a self in the internal and external sense media. Whatever sense of self that may surround these objects is not inherent in them, and is instead simply the result of one's own penchant for "I-making" and "my-making." Seeing the artificiality of "I-making" and "my-making" in this way helps lead to a sense of disenchantment with these "makings," thus helping to abandon any clinging associated with them.

Thus emptiness in this sense relates directly to the third of the three characteristics: not-self. However, just as the three characteristics are not radically separate from one another — everything stressful is for that reason not-self — the practical application of this sense of emptiness is not radically different from the first. As SN 12.15 points out, when one no longer latches onto any idea of "my self," one sees phenomena within and without simply as examples of stress arising and passing away. To practice meditation from this perspective — seeing each state of concentration as an example of stress arising and passing away — is to develop emptiness as a meditative dwelling.

Emptiness in its third meaning, as a type of awareness-release, is an application of emptiness in its second. MN 43 describes this state of concentration as follows: "There is the case where a monk — having gone into the wilderness, to the root of a tree, or into an empty dwelling — considers this: 'This is empty of self or of anything pertaining to self.'" It adds that this awareness-release is different from the awareness-release that results when one doesn't attend to any themes. Thus this state of concentration cannot be entirely equated with the emptiness as a meditative dwelling mentioned in this sutta. MN 106 further adds that if one frequently abides in the emptiness awareness-release, one may either attain the dimension of nothingness — one of the formless states — or be committed to the discernment that will lead to Awakening. The first of these two alternatives is another way in which emptiness as an awareness-release differs from emptiness as a meditative dwelling as defined in MN 121. However, because the standard definition of discernment is seeing phenomena in terms of the four noble truths, the second alternative — being committed to discernment — would apparently follow the same pattern suggested by SN 12.15, above. In other words, as one no longer perceives phenomena in terms of self, one tends to view them simply as examples of stress arising and passing away. So, again, this third meaning of emptiness, like the second, eventually leads in practice back to the first. As MN 43 notes, when one attains full awakening, the themeless awareness-release and the emptiness awareness-release come to differ only in name, and not in actuality.

In reading the following sutta, you will notice that the various meanings of emptiness will fit some contexts better than others. Still, it is important to remember that in the course of practice, all three meanings are related and all will inevitably play a role in Awakening.