Immortality

Immortality refers to the avoidance or reversion of death or an entity that appears to exist forever.

Immortality is one of the defining characteristics of the Q. The fact that the Q Continuum forced immortality onto other Q dismayed one of the Q who would attempt suicide. He claimed that immortality was one of the vulnerabilities of the Q.

The Greek gods were immortal. In order to end their existence, they spread themselves against the wind, becoming thinner until reaching a point where they could not reform into their bodies. 

The Douwd were immortal beings known for hiding their identity. 

While living on Earth, Flint possessed immortality in the form of instant tissue regeneration. He lost his immortality when he left to live on Holberg 917G. 

Zefram Cochrane was rejuvenated and made immortal by the Companion in 2119. He gave up his immortality to grow old and die with the Companion. 

The Gideon, in practice, were immortal for awhile between the elimination and reintroduction of germs on their planet, as death came to them only when their body couldn't regenerate itself due to disease. 

The planet Ba'ku released metaphasic radiation which caused cells of humanoids to be continuously regenerated, causing immortality. 

Dr. Bathkin and his successor Elias Giger attempted to make themselves immortal by developing a cellular regeneration and entertainment chamber. Giger eventually succeeded in creating this machine and shared his interest with Weyoun. 

Near immortality
Roger Korby believed that a person could have practical immortality by continuously transferring his or her consciousness into an android body. This was later offered to Uhura by Harry Mudd on the planet Mudd. 

There were many known entities with extraordinarily long lifespans that were considered virtually immortal. This includes the Redjac entity. 

Leonard McCoy considered the advanced age of Spock (and other Vulcans) would achieve to be close to immortality. 

Faux immortality
Trelane poetically stated that a kiss from Yeoman Teresa Ross would make him immortal. 

Work related to Sherlock Holmes was considered immortal by Jean-Luc Picard. 

According to Beverly Crusher, Starfleet captains always act as if they were immortal. 

It was hypothesized by Noonian Soong that procreation was done in part to give humanoids a sense of immortality. 

Onaya claimed her inspirations were beneficial. Although the ones she inspired died young, their works became "immortal." 

Lewis Zimmerman believed that by placing Julian Bashir's image on the Long-term Medical Holographic program, it would be a chance for Bashir to have "immortality." 

Benjamin Sisko and Weyoun agreed that Vorta cloning was basically immortality, of a sort. 

Although the Borg discarded drones once they were irreparably damaged, their memories continued to exist within the Borg Collective. Seven of Nine compared this to the Human concept of immortality, and it helped relieve any fear of death.