Double play is a term used in baseball to describe a situation where two defensive players are involved in a single play. It usually occurs when a runner is on first base and the batter hits a ground ball to the infield. The fielder must throw to second base to get the lead runner out, while the second baseman throws to first base to get the batter out. In this situation, both players are involved in the same play, thus it is referred to as a double play.