Star Trek: 10 Crazy Facts About The Trill
7. The Name Of The Hosts
On the planet of Trill, there are two species that evolved in an symbiotic relationship. The Trills play host to the symbionts, a state they both benefit from in some capacity. But there is an important distinction between them, specifically that the name "Trill" doesn't refer to both species.
Trill is the name that refers to the host species only, regardless of whether they are joined or not. Symbionts are the species that require hosting. But the fact that they were a joined species wasn't widely known until more than a century after the Trills started travelling through space. As such, the name Trill was unknowingly used to describe both species.
It was an incident on the Enterprise-D involving the Odan symbiont that illuminated the intricacies of Trill culture and the joining process. After that point, the reality of the Trills became more apparent and other races began to learn about their symbiotic nature.