Some sort of organization with rules that defines winners and losers. Chess tourneys are a sport, but playing a game of chess with a friend is not. This means cheerleading itself isn't a sport, but cheerleading tourneys make the act of "cheerleading" a sport.

Or kids racing down the block. If they did it once or twice, then it's a race. If they did it every friday with progressive heats and a clear winner, then it becomes sport.