The "Final Passing"
Nirvana is not death, for death entails rebirth. Nor is it annihilation, which entails a being or essence to annihilate. Just as enlightenment expels all ignorance, so nirvana entails the end of all suffering. Suffering is intimately connected to birth, old age, sickness, and death of all kinds. Nirvana is a literal [there is no equivalent word for it as it is neither thing, nor place, nor state]. It is not merely the absence of suffering.
Nirvana is to be experienced. Interestingly, it is not a noun (person, place, or thing) at all. Rather it is a
verb inasmuch as one is "nirvanered" (
nibbuti, cooled, calmed, quenched, cleansed). Just as there are phenomena in the world of daily experience, nirvana is the "unconditioned" and "deathless" element.

This is a deep and profound statement not amenable to
intellectualinvestigation. Fortunately, it will admit of direct experience. Those nevertheless wishing to approach the matter through intellect will find Bhikkhu Bodhi's exhaustive treatment of the subject of nirvana in "The Buddha's Teaching: As It Is" very helpful. (Bhikkhu Bodhi is the greatest Buddhist scholar alive today; the audio CD may be purchased or is available free from
BAUS.org).
"The Buddha's Teaching As It Is" by Bhikkhu Bodhi (MP3)
(BuddhaNet) Ten audio lectures on the fundamentals of the Buddha's Teaching by Ven. Bhikkhu Bodhi, an American Buddhist scholar-monk. Lecture 6: "Nibbana (Nirvana)."
Nirvana is a reality that may be seen and touched. To glimpse it is to attain stream-entry, the first stage of enlightenment. The consummation is full enlightenment, but that takes two forms. The first is "with remainder"; the second, "without remainder." That is, one attains nirvana and the
Five Aggregates continue. One is alive as before, experiencing the results and fruits (
vipaka and
phala) of karma without clinging to experience or identifying with it. Eventually, karma is exhausted, then at that point there is
parinirvana, or liberation without remainder, without aggregates, without any further coming to be. English hardly has words for it. But it may be understood by many positive analogies given by the Buddha, which are not applicable to death. For example, nirvana is the unsurpassable:
peace
happiness
bliss
refuge (sarana)
safety
When the Buddha Passed Away
Susan Elbaum Jootla (excerpts from
Teacher of the Devas, BPS)
Devas and
brahmas were active at several phases of the Great Passing — the Buddha's final entrance into nirvana away at Kusinara (modern Kushinagar, India) — as recorded in the
Maha Parinibbana Sutta (DN 16). This event was not just the demise of a greatly revered being but it also represented the personal consummation of his teachings. It was the utter, permanent cessation of the aggregates of the one who discovered and taught the way to the end of suffering.
A short while before the Buddha attained final nirvana, he lay down to rest between two sal-trees. They began flowering profusely, out of season. After some time, the Buddha dismissed the monk who had been fanning. Then the Venerable Ananda, his devoted attendant [and cousin], asked him why he asked the monk to go. The Buddha replied:

"Ananda, the
devas from ten world-spheres have gathered to see the Tathagata. For a distance of twelve
yojanas around the Mallas' sal-grove near Kusinara there is not a space you could touch with the point of a hair that is not filled with mighty
devas, and they are grumbling, 'We have come a long way to see the
Tathagata. It is rare for a Tathagata, a fully enlightened Buddha, to arise in the world, and tonight in the last watch the Tathagata will attain final nirvana, and this mighty monk is standing in front of the Venerable One, preventing us from getting a last glimpse of the Tathagata!'" (DN 16.5.5)
Ananda, who had standing permission to ask the Buddha anything, next wanted to know what kinds of devas were around them. The Buddha said he saw lower devas who are "weeping and tearing their hair" in distress, moaning, "All too soon the Blessed One is passing away, all too soon the Well-Farer is passing away, all too soon the Eye of the World is disappearing!" But there were also devas "free from craving" (i.e., enlightened) who endured this patiently, saying. "All compounded things are impermanent — what is the use of this?" (DN 16.5.6).
After passing through the eight meditative absorptions (
jhanas), the Buddha finally expired, attaining
parinirvana, the immutable cessation of rebirth. At that moment the Earth quaked, as it does whenever
buddhas pass away. The
brahma Sahampati, who had entreated the Buddha to teach forty-five years earlier, spoke a verse as a short eulogy: