Trill was an inhabited planet. It was the homeworld of two sentient species who shared a symbiotic relationship: the Trill, a warp-capable humanoid species, and the Trill symbionts, who were non-humanoid. (DS9: "Invasive Procedures", "Equilibrium")
In 2375, the USS Destiny was en route to Trill with the Dax symbiont, which had been injured, when the entity took a turn for the worse. As a result, it had to be joined with Ezri Tigan. (DS9: "Shadows and Symbols")
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Institutions and agenciesEdit
In the series bible for Star Trek: Deep Space Nine, it was said that the underground of Trill and the planet's surface, "many centuries ago," were where the Trill symbionts and humanoids lived, respectively. It was also stated that "an environmental disaster" led the two species to begin integrating with each other. 
As there were two make-up configurations designed for the Trill, Jadzia Dax actress Terry Farrell once suggested that "the north of Trill" was the birthplace of the simpler-looking Trill whereas the planet's south was where the more elaborate physical design, involving spots, originated. (Cinefantastique, Vol. 23, No. 6, p. 21) While filming DS9 Season 1 finale "In the Hands of the Prophets", Farrell also expressed hopes of seeing Trill in the second season of Star Trek: Deep Space Nine, saying, "I'd like to go to the planet where I'm from." (Cinefantastique, Vol. 24, Nos. 3/4, p. 98)
The name of the Trill homeworld was first established as "Trill" in the third season outing "Equilibrium", which also marks the first time the planet appeared on Star Trek: Deep Space Nine. Portraying the planet in that episode proved highly expensive. Some of the Trill exteriors shown in "Equilibrium" were filmed on location in Huntington Gardens. "I think there were a lot of interesting things about the Trill homeworld that you found out in the episode," commented René Echevarria, who wrote "Equilibrium"'s script. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, pp. 84 & 85)
Star Trek: Star Charts (p. 33) and the Star Trek Encyclopedia (4th ed., vol. 2, p. 421) designate the Trill homeworld as an M-class planet. Also according to Star Trek: Star Charts (p. 33), Trill (Trillius Prime) was governed by the Trill Symbiosis and was admitted into the Federation in 2285. The planet's capital was Mak'ala. Other points of interest included the Hoobishan Baths, the Tenurial Ice Cliffs, and the Caves of Mak'ala. As of 2370, the planet was populated by 650 million Trill and 11 million symbionts, though the existence of symbionts was not widely known prior to 2367.
According to Star Trek: Star Charts (pp. 36 & 46) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Trill system was located in the Alpha Quadrant. This was a binary system with a G-class star and a K-class star. The system was a destination on an Earth trade route in the 22nd century and on a major space lane in the late 24th century.