Death

From Memory Alpha, the free Star Trek reference.

"Every life comes to an end when time demands it. Loss of life is to be mourned, but only if the life was wasted."

Death is the permanent end of all life functions in a lifeform (or part of a lifeform in the case of tissue damage). It can also mean the absence of life or state of being dead. Many cultures address this process as it applies to sentient beings in spiritual terms, including the holding of wakes, memorials, and funerals following the death of an individual.

The study of death is called thanatology.

Artificial lifeforms can cease functioning through induced self-destruction.

For most lifeforms, death is a permanent state. However, sometimes resuscitation is possible:

In 2267, Scotty was killed by the Nomad probe. Nomad subsequently resurrected him.
In the same year, Leonard McCoy was killed by a knight's lance, in an environment that created reality from people's thoughts. McCoy's resurrection was brought about by the power which created the knight in the first place, after Kirk realizes what is going on.
In 2285, Spock experienced death, but with his katra having been placed into Leonard McCoy and his body regenerated on the Genesis Planet, he was able to live again. (Star Trek III: The Search for Spock)
Tasha Yar killed Yareena with a poisoned, spiked weapon. Dr. Crusher revived her. The details of how this was achieved weren't described. ("Code of Honor")

Neelix was also brought back from being dead, but as his death did not match his culture's beliefs regarding an afterlife, he was profoundly disturbed by the experience. (VOY: "Mortal Coil")
The Kobali were capable of reanimating dead lifeforms in order to procreate their species, however the reanimated corpse would have a limited memory of his or her prior life. (VOY: "Ashes to Ashes")
Data has noted that his off-switch provides for an experience comparable to death, albeit one that permits a simple resurrection. (TNG: "The Schizoid Man")
In 2369, upon his death, Kobliad criminal Rao Vantika transferred his consciousness to Doctor Julian Bashir and continued living in Bashir. (DS9: "The Passenger")

Death is sometimes perceived as an event that is common to all living things. In 2366, Captain Jean-Luc Picard used death and his own mortality as a way to show Nuria and her people that he was no different than they were, and certainly not a god they named "the Picard". (TNG: "Who Watches The Watchers")

Death may also be seen as a personification, attributed to a single entity, figure, or symbol. In thsi form, Death is known by many names, one of the more common being the "Grim Reaper".

In 2267, Lazarus equated his parallel universe counterpart as "Death" and "Anti-Life" (TOS: "The Alternative Factor"). A century later, in 2370, the USS Enterprise D encountered an alien probe which began to transform the Enterprise into an alien city, complete with hieroglyphic symbols, one of which was the symbol for Death (TNG: "Masks").

Actor William Sadler, who played the character of Luther Sloan in Star Trek: Deep Space Nine, portrayed Death in Bill & Ted's Bogus Journey; part of which was filmed at Vasquez Rocks, a filming site used in several Star Trek productions.