The small, long-lived, intelligent creatures of the Trill homeworld that are implanted in humanoid Trill hosts, carrying the memories of every previous host from one lifetime to the next.
Symbionts are helpless-looking, slug-like beings that can live for many centuries. They breed in the underground milky pools of the Caves of Mak'ala on Trill, tended by unjoined Guardians, and communicate with one another by discharges of energy. First shown in The Next Generation episode The Host, they are explored mainly through the Dax symbiont in Deep Space Nine.
Once a symbiont is placed in a host, the two become one blended personality within days, and after about ninety-three hours neither can survive separation. When the host dies the symbiont passes to a new one. Trill law forbids rejoined hosts from resuming old romances, a taboo seen in Rejoined.
Equilibrium reveals that far more Trill are suitable hosts than the Symbiosis Commission admits, a secret kept to protect the few symbionts. Invasive Procedures and Facets also turn on their nature.