What does deathless mean?

To break through to even so much as Metta (Lovingkindness), is a taste of the deathless (for as long as it's sustained).

So all these definitions above are helpful, but you can't truly know what the deathless is until you practice the 8-fold path enough that a Brahmavihara (or higher) occurs as a resultant. Experiential knowledge is "Knowledge" in Buddhism, not mere book-reading, intellectual-level "knowledge".

I feel it's important for this distinction to be mentioned. It's a feeling (which is "not of the flesh", as opposed to a "worldly feeling"), and not a concept.

Note: I'm a Buddhist monk, however I wish to remain anonymous. I am bound by Vinaya to not discuss any attainment's I might have, so please don't ask (if "I've ever experienced the deathless").

The deathless element refers to the aspect of your existence which is persistent through all mental states and conditions. It is not death in the sense of physical life or death, but in the sense of being eternally persistent within all conscious existence.

In this interpretation, it shows that Nibbana is not a state to be attained, but is instead an element of being, which is to be recognized as ever-present and then aligned with. Through recognizing yourself only as the deathless element, and only as a witness to the conditioned phenomenon, you reach the state of Nibbana.

There's an explanation on page 36 (page 10 of the PDF file) of Pita Tan's introduction to SN 56.11. The context is explaining the various types of dukkha ("birth is dukkha, death is dukkha, ..." and so on).

When we understand this, we have a good idea why nirvana—the final state of the Buddha and the arhats—are also “described” (for the convenience of the understanding of the unawakened) as being the opposites of birth, decay, disease and death—nirvana is thus non-birth, non-decay, non-disease and nondeath, that is to say:

• ajāta or abhūta “birth-free” being unborn, being free of rebirth, hence, no redeath;
• avyādhi or ajarā “decay-free” not subject to change, bodily or mental, or disease and dis-ease;
• asoka or abhaya “fear-free” without craving, there is no fear, nor danger from the world;
• amara or amata “death-free” without rebirth, there is no redeath, the cycle is finally broken.

I addressed this in my blog recently. Quotation follows:

The Buddha teaches the path of the arhant, the Noble Eightfold Path, consisting of four levels of attainment of degrees of progressive realization or accomplishment: stream entrant, whereby one achieves emancipation within seven rebirths without ever experiencing a lower non-human rebirth; a once-returner, whereby one is reborn as a human being once more only; a non-returner, whereby one is never again reborn as a human being, but will be born as a deva being in the Five Pure Abodes; or an arhant, in which on death one achieves parinibbana (lit. ‘complete snuffing out,’ as of a fire) and subsequent ‘immortality’ (amata). This Pali word, amata, pervades the Pali Canon, and is generally translated as ‘deathlessness,’ but the PED makes it clear that the primary meaning is amrita, translated by Rhys Davids as ‘ambrosia’ or ‘water of immorality,’ from the Sanskrit root MR, ‘death.’ Amrita, which is also described in the PED as a “medicine,” is the Buddhist word for soma, the Vedic mind-altering beverage that is identical with the plant-based psychedelic that was worshipped by the early Aryans and inspired the rishis, including the Seven Sages, to write the ecstatic hymns of the Rig Veda. Mata means both ‘thought’ and ‘dead,’ so the state of amata is a state beyond both death and thinking. Paradoxically, a state of apparent ‘impercipience’ is identical with the realization of essential sentience itself. Hence, ‘deathlessness.’

http://palisuttas.com/2015/06/07/epitome-of-the-pali-canon/

My interpretation of the Buddha's teachings (with input from many teachers) is that he was not especially concerned with what came before or after this existence. He said that he taught "the end of suffering" and little else.

As someone said earlier, nibbana is the cessation of the need for rebirth, thus any death. The way I understand it is that the conditions for some following existence cease, like blowing out a candle, and there is just nothing. The aspect of some sort of permanent "bliss" is counter to the teachings, in which nothing is permanent. I believe that permanence aspect was added on later because followers could not let go of that need to continue on in some respect.

In Buddhism, to be deathless means you have realized Nirvana and are liberated from samsara and will have no more rebirths and thus no more deaths. This does not mean you cease to exist for that would be nihilism and the Budha was clear that to say his teachings were nihilist was heresy.

Lanka's answer strikes me as comprehensive. Except for the etymology of the word. Pali amata and Sanskrit amṛta derive from the root √mṛ 'die'. The form here is a past participle used as adjective - mṛta means 'died, dead'; compounded with the 'a', amṛta means 'undying, deathless'.

Ultimately it probably comes from Vedic, as the Upaniṣads more frequently speak of the cyclic afterlife as punarmṛta 're-death' or punarmṛtyu 'repeated dying'. It's this repeated death that makes the fact of cyclic existence ultimately unbearable and motivates the search for the deathless. And the opposite of repeated death is no death, the deathless. It can mean "immortality" in Vedic, in the sense that one's ātman exists permanently and once one transcends repeated death, then ātman rejoins and attains union with the universal principle of being, Brahman. Notably in the Ariyapariyesana Sutta it is Brahmā, the personification of Brahman, who urges the Buddha to teach after he opens the doors to the deathless.

But as Lanka says, in Buddhism it is one of many synonyms for nibbāna.

"Deathless" typically translates amrita (lit. "no-death"), also translated as "the nectar", using the useful parallel with Greek mythology. Another translation of "to attain deathless" is "to partake of the nectar" - just another metaphorical way to refer to The Fruit. I suppose we could compare this with the modern American idiom "drank the Kool-Aid" - although the latter has rather satirical connotations.

More specifically "the deathless aspect" (amrita-dhatu) is (seems to be?) a synonym with what Mahayana calls "the absolute truth" -- the perspective of Three Marks Of Existence, Emptiness, and Pratityasamutpada -- as opposed to "the conventional truth" -- the perspective of Rebirth, Karma, and Samsara vs. Nirvana dichotomy.