Humanoids of the planet Trill, a few of whom are joined with long-lived symbionts that carry the memories of every previous host.
The Trill are a humanoid species from the planet Trill in the Alpha Quadrant, recognisable by the spots that run down the sides of their bodies. They share their world with a second species, the symbionts, small long-lived creatures that live in pools beneath the surface.
A small minority of Trill are selected through a rigorous process to be joined with a symbiont. The joined Trill gains the memories, skills and experiences of every previous host, while keeping a personality of their own. Joining is treated as a great honour, and Trill society surrounds it with strict rules and rituals.
The Trill were introduced in The Host, in which the symbiont Odan was briefly carried by Riker. On Deep Space Nine, Jadzia Dax and later Ezri Dax carried the Dax symbiont, whose former hosts included Curzon Dax. Joined Trill also appear in later series, including Discovery.