Nirvana is a Sanskrit term that is interpreted in various ways: 1) Cessation (Nirodha), or extinction, referring to the elimination of the afflictions (klesha) at the time of enlightenment or to the ceasing to be of the skandhas (see Five Skandhas) when an enlightened person at death chooses to be reborn no longer; 2) freedom from desire (greed or [lust]] - raga); and 3) no longer either coming into being or ceasing to be.
In addition to the above references to enlightenment, in later times the term Nirvana came to be used as a polite way of speaking of the death of a monk (Bhikshu) or nun (Bhikshuni) regardless of whether or not he was enlightened and truly entering Nirvana.
“But if someone should ask you, Vaccha: 'This fire in front of you that is extinguished, in what direction has that fire Gone from here, east, west, north or south?' What would you answer to such a question?”
“That does not apply, dear Gotama. For that fire that burned because of fuel consisting of straw and wood, has consumed this and not been given anything else and is therefore called 'extinguished (nisbuto) through lack of fuel.'”
“Just so the form of the Thus Come One is given up,its root broken, uprooted like a palm, free from further growth or renewed existence in the future. The Thus Come One is free from everything called form, he is deep immeAsurable, unfathomable, just like the deep ocean.” (Majjhima NiKaya I 486ff)
The Buddha repeats the whole series of questions and answers, substituting the other four of the Five Skandhas–feeling, cognition, formations, and consciousness–one by one in each successive repetition.
“Most people think that Nirvana follows upon death, but actually it is not necessarily an after-death state. It is the Certification of the attainment of noumenal being (li). 'Nirvana' is a Sanskrit word, which is interpreted to mean 'neither coming into being nor ceasing to be'. Since there is neither coming into being nor ceasing to be, birth and death have come to an end. One attains Nirvana when one is no longer subject to birth and death. However, Nirvana does not mean that the Buddha dies. When the Buddha dies, he enters Nirvana; he enters the noumenal being of Nirvana and verifies its four qualities–permanence, bliss, true self, and purity. Some people who haven't seen things clearly in their study of Buddhism think that Nirvana is just death, but Nirvana is definitely not death. One who has that view does not understand the principles of Buddhism.” (SS I 180-181)
samsara [i.e., the stream of conditioned existence has nothing that distinguishes it from Nirvana; Nirvana has nothing that distinguishes it from samsara. The limit of Nirvana is the limit of samsara; there is not even the subtlest something sepaRating the two.
World-Honored One, the ground of fruition is Bodhi, Nirvana, true suchness, the Buddha Nature, the amala-consciousness, the empty Treasury of the Thus Come One, the Great, perfect mirror-wisdom. But although it is called by these seven names, it is pure and perfect, its substance is durable, like royal Vajra, everlasting and in[[destructible]]. (SS IV]] 207)