Afterlife is a term referring to existence after one's death.
Some Humans believed that a person would be sent to a different afterlife, either Heaven or Hell, depending on their actions in life. (TOS: "Space Seed") In 2365, Captain Picard explained that there were two contemporary philosophies on afterlife. One is the belief that death is the transformation into an indestructible and unchanging form. The purpose of the entire universe is to then maintain that form in an Garden of Eden-like place. The other is that death is simply blinking into nothingness. Picard himself believed that the existence of lifeforms is part of a reality beyond what was currently understood as reality and therefore both philosophies were insufficient. (TNG: "Where Silence Has Lease")
Many cultures believe that people will exist in the afterlife in a "pure" physical state, cured of health issues from their previous life. In 2368, when Geordi La Forge and Ro Laren were thrown out of phase by a transporter accident and declared dead by a crew that could not see them, the blind La Forge regarded the continued function of his VISOR as proof that, contrary to what Ro believed, they were still alive. (TNG: "The Next Phase")
In 2369, after a failure of his artificial heart, Jean-Luc Picard experienced being sent to an afterlife controlled by Q. Q eventually decided to return Picard to life. (TNG: "Tapestry") A species of non-corporeal lifeforms who fed on the consciousness of corporeal beings used a Matrix to trap minds of people who were close to death. The aliens needed the person to enter the Matrix voluntarily, so they claimed it was the afterlife. (VOY: "Coda")
Klingon mythology taught that honorable warriors are sent to Sto-vo-kor when they die, while dishonorable ones are sent to Gre'thor. B'Elanna Torres experienced being sent to Gre'thor to save her mother from dishonor. (VOY: "Barge of the Dead")
Talaxians mythology has a belief in an afterlife known as the Great Forest, in which a great tree stands at the center. It is said that upon death, a Talaxian goes to the afterlife and meets the souls of his deceased family upon arrival. (VOY: "Mortal Coil")