The Buddha’s description of Nirvana
“Being myself subject to birth, ageing, ailment, death, sorrow, and defilement, seeing danger in what is subject to those things and seeking the unborn, unageing, unailing, deathless, sorrowless, undefiled supreme surcease of bondage, Nibbana (Sanskrit: nirvana), I attained it. The knowledge and vision arose in me: ‘My deliverance is unassailable; this is my last birth; there is no renewal of being.’” M.26 (translated by Nyanamoli).
The Buddha describes Nirvana as extinction (possibly blowing out) of the phases of actual being, i.e. of life as dynamic process, that is to say, of the dynamic (hence energised, hence ‘enlightened’ or ‘enlightening’) interaction of the constituents of life (none of which are atman (or atta)), indeed of dynamism as such.
It is interesting to note that the Buddha does not state for how long he attained Nibbana the first time. Since the Buddha became old, got sick and died, that suggests that he did not abide (for more than a moment, or a short period, though these may have been repeated) in Nibbana.
If dynamism is understood as (unceasing) activity, then dynamism comes to end in rest (i.e. in non-activity, to wit, death). This in turn suggests that the word nirvana was taken to mean @ rest (and which it was believed was the pre-state (that is to say, the inactive potential) of atman).
Since the Buddha claimed to have achieved nirvana – though he never did prove his claim, i.e. @ rest status (in this life), there emerged two distinct nirvanas, namely
1. particular (hence relative, hence momentary) @rest status
2. and non-particular (i.e. potential, or ground condition, hence non-relative (hence not-in-time) = absolute) @rest’ness, called pari-nirvana (or nibbana).