What is the term for the Buddha's final passing away?

The Buddha’s final passing away is called parinirvana.

In Buddhist vocabulary, parinirvana refers to the death of someone who has already attained nirvana during life. For the historical Buddha, it describes his death at Kushinagar, traditionally after decades of teaching and at the age of 80. The event is also called the Buddha’s mahaparinirvana, or “great final nirvana.”

The distinction from nirvana is important. Nirvana can describe liberation from craving, suffering and rebirth achieved during life; parinirvana refers to what follows the physical death of a fully liberated being. Buddhist traditions explain this as the end of the conditions associated with continued rebirth.

A common mistake is treating parinirvana as simply another word for enlightenment. Enlightenment occurred at Bodh Gaya when Siddhartha became the Buddha. His parinirvana occurred much later at Kushinagar, traditionally represented in art by a reclining Buddha surrounded by disciples.