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.
In the late 24th century, a 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)