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.
Ball-in-play force plays[]
When the batter has put the ball into play, the batter becomes a runner, forcing the runner at first, and any other runner who does not have an empty base behind him. A forced runner's force base is the next base beyond his time-of-pitch base. Any attempt by fielders to put a forced runner out is called a force play. A forced runner is out (called a force out) when a fielder with the ball touches the runner's force base before the runner reaches that base. A forced runner also may be tagged out in the usual fashion as well; such a tag is still considered a force play if the tag is made before the runner reaches his force base. Any play on the batter-runner before he reaches first base is the same as a force play, though the rules technically do not include this in the definition of a force play.
(Note that, by rulebook definition, when a runner is put out after the catch of a fly ball, this is not a force play, even though the runner can be put out in the same manner.)
Once a forced runner reaches the next base, the force is off of that runner. If the runner attempts to gain additional bases, the runner can be put out only by tagging. An attempt by the batter to advance past first base does not put additional force on a runner.
A force on a runner is "removed" when the batter or a following runner is put out. This most often happens on fly outs; on such, the batter-runner is out, and the other runner(s) usually return to their time-of-pitch base because they are no longer forced to advance. A notable and often-misunderstood play is the situation where a runner is on first base, and the batter hits a ground ball that is fielded by the first baseman. Even though he might be right next to the base, the first basemen will often throw to second base to put out the runner from first, and then wait for the throw back. The reason is that if he steps on first base for the force out, the force is taken off the runner, and then the fielder taking the throw at second then has to tag the runner to complete the double play, which is a more difficult action then simply stepping on the base. If the first basement doesn't start the play by taking the out at first base, then the play at second base will be a force out.
Other force plays[]
When the batter is entitled to advance to first base without liability to be put out (base on balls, hit by pitch, catcher's interference, etc.), if first base is occupied, that runner is forced to advance to second base to make room at first for the batter. If there is also a runner at second, that runner must advance to third base, and so on. In such a situation, the runners who are forced are entitled to advance without being put out. The same rule applies if a runner is awarded a base. For example, if a runner at second base is awarded third base due to an obstruction call, but third base is occupied, the runner at third is entitled to advance to home plate, scoring a run.
Scoring on force outs[]
No run can be scored during the same continuous playing action as a force out for the third out, even if a runner reaches home plate before the third out is recorded. As a result, on a batted ball with two outs, fielders will nearly always ignore a runner trying to score, attempting instead to force out the batter or another runner.
An appeal play may also be a force play; for example, with runners on first and third bases and two out, the batter gets a hit but the runner from first misses second base on the way to third. After a proper appeal, this runner will be called out. This is a force out because the runner was out for failing to touch a base to which he was forced; this force out is the third out and thus the run does not score. However, most appeals are not force plays, because appeals usually do not involve a forced runner.