A turn is a time period during which one or more of the players in a game is allowed to act (issue commands to units, construct buildings, engage in diplomacy, etc.). All of the Civilization, Call to Power, and Colonization games and Sid Meier's Alpha Centauri are turn-based strategy games, meaning that they use turns to measure the passage of time.
In multiplayer games that include both human and AI players, the humans take their turns first. Multiplayer games may also allow the host to toggle simultaneous turns on or off and set a turn timer, which limits the amount of time players have to think and act before the game progresses.