In baseball, an assist (denoted by A) is a defensive statistic, baseball being the rare sport in which the defensive team controls the ball. An assist is awarded to every defensive player who fields or touches the ball (after it has been hit by the batter) prior to the recording of a putout, even if the contact was unintentional. For example, if a ball strikes a player's leg and bounces off him to another fielder, who tags the baserunner, the first player is credited with an assist. A fielder can receive a maximum of one assist per out recorded. An assist is also awarded if a putout would have occurred, had not another fielder committed an error. An example of such a situation would be when the shorstop fields the ball cleanly, but the first baseman drops his throw. In this case, the first baseman would get an error but the shortstop would still get an assist.
If a pitcher records a strikeout and the catcher catches the third strike, the pitcher is not credited with an assist. However, if the batter becomes a baserunner on a dropped third strike and the pitcher is involved in gaining a putout (i.e., he fields the ball and throws to first base), the pitcher is credited with an assist just as any other fielder would be.
Assists are an important statistic for outfielders, as a play often occurs when a baserunner on the opposing team attempts to advance on the basepaths when the ball is hit to the outfield (even on a caught fly ball that results in an out; see tag up). It is the outfielder's job to field the ball and make an accurate throw to another fielder who is covering the base before the runner reaches it. The fielder then attempts to tag the runner out. This is especially important if the runner was trying to reach home plate, as this assist prevents the baserunner from scoring a run. Assists are much rarer for outfielders than infielders because the play is harder to make, and because outfielder assist situations occur less often than the traditional ground-ball assist for infielders. However, as a result, outfield assists are worth far more than infield assists, and tell more about an outfielder's throwing arm than infielder assists do.
In recent years, some sabermetricians have begun referring to assists by outfielders as baserunner kills. Some sabermetricians are also using baserunner holds as a statistic to measure outfield arms . A baserunner hold occurs when the baserunner is prevented from taking an extra base. This can be combined with baserunner kills for better accuracy, as runners often do not try for an extra base when an outfielder with an excellent arm is playing.
- MLB.com - MLB Official Rules - The Official Scorer