An orbit is one revolution of the dealer button around the table in a poker game. An orbit represents one entire cycle of the game, and is a common measure of length of play (e.g. "I'll stick around for one more orbit, then I've gotta go home").
In a nine-handed game, an orbit is nine hands. In a six-handed game, an orbit is six hands.
The term naturally only applies to variants that use a dealer button, such as Hold 'em. Games without such a button (like 7-Stud) merely measure length of a game by hands.