A serial loop occurs when a player takes a meaningful action which requires the action of another player to determine a win, but whose resolution is not tied to real time. In simpler terms, this means taking turns in most forms of board games and some strategy games. It also covers actions such as sending requests in social games.
Serial loops are synchronous but atemporal. Play based on serial loops is often described (slightly inaccurately) as turn-based. It often suffers from the problem of players either forgetting to take their turn or choosing not to finish a game that has reached its loss point. Such games therefore need to allow players to play many concurrent games in order for the experience to be satisfying.