When I contemplate using Terran in my own scifi work, its as a throwback to its literary form. The only Terran I can think of, video game wise, is Starcraft - a game I've never actually played. I'm sure its been used elsewhere though, since it makes perfect sense: Terran is both a noun and an adjective, derived from Middle English terrene, which means "of the earth", secular, temporal, and all that good stuff.
