In baseball, a force play occurs when a defensive player fields or receives a ground ball and steps on or touches a base ahead of a runner who has been forced to advance toward that base. The result of the play is a putout of the baserunner.
In order for a play to qualify as a "force", a baserunner must either be advancing toward first base after hitting a ground ball or must have no option but to advance to the next base. A baserunner must advance to the next base when all previous bases are occupied by another baserunner or, in the case of being at first base, when the player at bat hits a ground ball (and becomes a runner). The play is described as a "force" because the runner is compelled to advance to the next base, having no other options. If the batter or the runner on a previous base is put out, there is no longer a force on the runner ahead. This often occurs in the case of a fly out, which removes the force, but requires the runner to return to his original base to "tag up" if he intends to advance. A runner subject to a force play can still be tagged out by a defensive player if he is tagged before reaching the next base.