Space duty was any posting that took a Starfleet crewmember into space.
In 2293, Tuvok's formal protest of Captain Hikaru Sulu's mounting a rescue mission to rescue James T. Kirk and Leonard McCoy was met with surprise by Sulu, who quipped, "A pretty bold statement for an Ensign with only two months space duty under his belt." (VOY: "Flashback")
In 2364, Data told his brother Lore that if he wished to acquire a Starfleet uniform like Data's, it would necessitate "four years at the Academy, another three as ensign, ten or twelve on varied space duty in the lieutenant grades." (TNG: "Datalore")