Who is ignorant and when does one realize that one is ignorant?

Can one know nothing of ignorance and still become enlightened?

This is a very big epistemological question.

Ignorance is the state of being unaware, or uninformed.

Enlightenment is the state of being informed.

Knowing ignorance or knowing nothing of ignorance are posited opposites of non-existent states. Probably the answer is no.

However; take the Rumsfeldian view of knowledge - there are known knowns, and known unknowns. We know that we know things, and we know there are things we don't know - these are the states of knowledge everyone understands. There could also be unknown knowns and unknown unknowns. I have yet to imagine a scenario for unknown unknowns - that's like proving the existence of a negative / non-existent thing. Perhaps unknown unknown knowns.... ugh.

Stay with me- for the case of unknown knowns - there may be things we would know if we were made aware of the possibility they could be known. A case that comes to mind is say you have knowledge in one science that people in another field of science don't realize exists. Phlogiston perhaps or the ether. All I need to do is tell you the knowledge you are seeking exists in another field of science. I don't say where and I don't say what.

Now you are in a state of knowing you are ignorant about that knowledge, and you may become enlightened.

Who is considered to be ignorant according to Buddhism?

I think that asking "who?" can get complicated because of anatta.

And there are different forms or schools of Buddhism, which so far as I know may have slightly understandings or definitions of ignorance and enlightenment etc.

Still, if you look at the four stages of enlightenment in Theravada Buddhism, or look at the Sutta Pitaka's list of ten fetters, the "fetter" of "ignorance" is eradicated at the fourth stage: i.e. that of Arahant, a fully awakened person.

I think that implies that anyone but an Arahant may be subject to ignorance (isn't fully knowledgeable).

What does the term ignorance cover? When does one realize, one is ignorant?

The Pali word for "ignorant" is avijjā.

That word starts with the prefix "a-" which means "without" (that prefix has the same meaning in Greek and in some English words, e.g. "atheist", "agnostic", "anhydrous", etc.).

Many Pali words used in Buddhism are like that (i.e. beginning with "a-") and IMO that sense is lost when it's translated into English.

Look at the opening line of the chant of metta for example:

Aham avero homi
May I be free from enmity and danger

A literal translation is ...

• Aham -- I (first person singular)
• avero -- if vera means "hatred, revenge, hostile action", so avera means "absence of enmity, friendliness"
• homi -- I should be (first person singular imperative of the verb 'to be')

... "I should be without-emnity".

I like to think that's clearer, a translation closer to what the original actually said.

The English word is often phrased as if the noun is a something (e.g. "freedom" or "ignorance") whereas the Pali word is often phrased as a not-something (e.g. "not-emnity" or "not-knowledge").

So anyway, when you ask, "what is ignorance?", then the answer is, "it's not-knowledge"; so the next question then is presumably, "what is 'knowledge'?"

If you ask "what is knowledge?", then you get an answer like Thiago's which talks about 'Right View'.

There are other types of knowledge (maybe called 'higher knowledge') too, e.g. as described in the Sekha-patipada Sutta:

he recollects his manifold past lives ... he sees beings passing away & re-appearing ... he enters & remains in the fermentation-free awareness-release & discernment-release

Can one know nothing of ignorance and still become enlightened?

The definition of ignorance as a "lack of knowledge" might suggest not -- even if a being is (at least temporarily) happy or at peace or humble, maybe that wouldn't be called "enlightened" or "becoming enlightened".

I'm thinking of Phil's snake as an example. Not from the Theravada tradition, there is a famous koan, called "Joshu's Dog". Maybe it's the potential to be enlightened that is what they call 'Buddha-nature'.

"Who is considered to be ignorant according to Buddhism? What does the term ignorance cover ?"

In the Sammaditthi Sutta: The Discourse on Right View:

"And what is ignorance, what is the origin of ignorance, what is the cessation of ignorance, what is the way leading to the cessation of ignorance?

Not knowing about dukkha, not knowing about the origin of dukkha, not knowing about the cessation of dukkha, not knowing about the way leading to the cessation of dukkha — this is called ignorance.

"When does one realize, one is ignorant ?"

I guess, when one realizes one does not understand / is unskillful in the above.

"Can one know nothing of ignorance and still become enlightened?"

As far as enlightenment is understood as nirvana (which is understood as cessation of dukkha), and ignorance is understood as the above, no. That would be the same as asking if someone who knows nothing of arithmetic to be able to sum numbers: his sum would denounce the very knowledge of arithmetic.

In the same way, someone skillful and wise on origin and cessation of dukkha, someone who put an end on dukkha necessarily is someone who is not an ignorant on the origin and cessation of dukkha.

everyone who is subject to samsara or the round of rebirths is ignorant, because

From ignorance as a requisite condition come fabrications.
From fabrications as a requisite condition comes consciousness.
From consciousness as a requisite condition comes name-&-form. From name-&-form as a requisite condition come the six sense media.
From the six sense media as a requisite condition comes contact.
Fromc ontact as a requisite condition comes feeling.
From feeling as a requisite condition comes craving.
From craving as a requisite condition comes clinging.
From clinging as a requisite condition comes becoming.
From becoming as a requisite condition comes birth.
From birth as a requisite condition, then aging & death, sorrow, lamentation, pain, distress, & despair come into play.

Paticcasamuppadavibhanga sutta ( SN 12.2)

i think ignorance covers nescience of the phenomenon of suffering and the way of escape from it

When does one realize, one is ignorant ?

perhaps once one realizes that they suffer while having no clue how to avoid that

Can one know nothing of ignorance and still become enlightened?

i find this an interesting question, my guess is 'NO', but i'm not sure i can explain why i think so and can't remember suttas which explain this point.
that's as far as awakening is concerned

as for nibbana i'm a bit more certain, just as suffering is effected by a chain of causes, quoted above, so is nibbana, for example

What is impermanent is suffering. What is suffering is nonself. What is nonself should be seen as it really is with correct wisdom thus: ‘This is not mine, this I am not, this is not my self.’
When one sees this thus as it really is with correct wisdom, the mind becomes dispassionate and is liberated from the taints by nonclinging.
By being liberated, it is steady;
by being steady, it is content;
by being content, he is not agitated.
Being unagitated, he personally attains Nibbāna.

Anicca sutta (SN 22.45)

an aspirant needs to go through a series of stages of mind development to arrive at that destination and those cannot be bypassed

the first step in curing an illness is its acknowledgement