Nirvāṇa marks the end of Saṃsāra, the latter term translatable with adequate accuracy as a ‘Disoriented Search’. Nothing more is to be said. Nirvāṇa is defined only in relationship to what it is not.
The answer to the question: ‘What is Nirvāṇa?’ lies in an understanding of the misunderstanding that underlies the question itself.
The self-scuttling has to be done at the level of the questioner. And to properly sunder the Self-Loop is to comprehensively answer the question.
It is dangerously facile to talk about the possible absence of an ‘Independent ‘Self” to one firmly ensconced in it. It is markedly unwise to try and explain the nature of ‘World’ to one who can interpret the explanation only from the platform of a presumed observing and separated ‘Self’.
[In other words, don’t write Sites like this one. It’s what the ‘Diamond Sūtra tries to do recklessly risking all credibility.]
Nirvāṇa in its proper definition has nothing at all to do with any empyrean ecstasy, cosmic peace or any of that later rubbish. And no, upon reaching it you still will not be able to part the Red Sea.
The word Nirvāṇa long predates the Buddhist literature. And the problem of ambitious Guru’s unknown to Shūnyam, who have pinned a plethora of enticing and outright misleading attributes on Nirvāṇa is a very old one.
The word Nirvāṇa, literally a ‘Flaring-Out’, has its etymological roots in a fire that has ‘Come to Rest’. In its earliest Buddhist elaboration, the MadhimaNikaya says it is like asking the direction taken by a dead fire: ‘To ask: ‘In which direction has [the dead] fire gone?’, is a question that: ‘does not fit the case’.
In the common analogy, its like explaining life outside water to a fish that has known nothing else and cannot conceive it with any credence.
The fish is an easier case. With us humans, explanation is both unconvincing and deleterious. What we conventionally mean by ‘Understanding’ itself begins in the presumption of a separated ”Self’ and World’.
It’s sort of like the situation at the counter at the Rolls-Royce dealership. If you need to ask the price you probably can’t afford it.
If you need to have Nirvāṇa explained, you won’t understand it.