Io (aka Sol Va) was a natural satellite, or moon, of Jupiter.
Io was the innermost of the four so-called Galilean moons (the others were Callisto, Europa and Ganymede) discovered by Galileo Galilei in the 17th century. With a diameter of 3,642 km, it is the fourth-largest moon in the Sol system.
Microbial life existed on Io. This was known to the Vulcan officers stationed on a secret surveillance base on Luna in the late 20th and early 21st centuries. (The Federation Files: "No Good Deed")
Io had only one Earth colony, devoted to mining. It had a population of 900,000. (Star Fleet Universe: Gazetteer Based on F&E Map, 14 August 2012 revision)
Namesake starships[]
In the late 24th century, the Luna class of starships -- named after natural satellites in the Sol system -- was commissioned. One of these was the USS Io, launched in 2379 under the command of Captain T'Vrea. (TNG novels Articles of the Federation and A Singular Destiny)