The home of the Trill, a symbiotic species whose joined hosts carry memories across lifetimes.
Trill is the homeworld of a joined species. Some Trill carry a long-lived symbiont that lives inside a host body, and the joined person shares the memories and personality of all of the symbiont's previous hosts. Unjoined symbionts live in the pools of the Caves of Mak'ala, and the Symbiosis Commission decides who will be joined.
The Trill were introduced in the Next Generation episode The Host, in which Beverly Crusher falls in love with the ambassador Odan. In Deep Space Nine, Jadzia Dax is a joined Trill, and after her death the Dax symbiont passes to Ezri. In Equilibrium, Jadzia visits Trill and learns that the Commission had hidden an earlier violent host, Joran, and the fact that far more Trill can be joined than it admits.
In Rejoined, Jadzia meets Lenara Kahn, whose symbiont was once married to one of Dax's hosts, a reunion Trill custom forbids. Discovery returns to Trill in the thirty-second century, where Adira Tal is joined with the Tal symbiont.