Force play

In baseball, a force is a situation when a baserunner is compelled (or forced) to vacate his time-of-pitch base—and thus try to advance to the next base—because the batter became a runner. A runner at first base is always forced to attempt to advance to second base when the batter becomes a runner. Runners at second or third base are forced only when all bases preceding their time-of-pitch base are occupied by other baserunners and the batter becomes a runner.