Trillius Prime was the inhabited prime planet of the Trillius system. This system was located less than 90 light years from the Sol system. (ENT: "Broken Bow", "Two Days and Two Nights")
Trillius Prime was one of the regular destinations of the Earth Cargo Service ships in the 22nd century. Travis Mayweather visited this planet in the sixth grade. It took three years for the ECS Horizon to get there. (ENT: "Broken Bow")
In "Two Days and Two Nights", the Enterprise, a NX-class starship, set a milestone in 2151 by becoming the first Earth ship to travel 90 light years from its homeworld. Trillius Prime was visited prior to this event.
According to "Horizon" the top speed of the ECS Horizon was warp 2, which corresponded to 8 times the speed of light according to the warp chart seen in "First Flight". This suggests that Trillius Prime was located about 24 light years away from Earth.
According to Star Trek: Star Charts (page 33), Trillius Prime was another name for the planet Trill.