A pick-off is a play in which a baserunner is put out when straying too far from the base he is occupying. A pick-off cannot happen on a batted ball; it either takes place before or after a pitch is thrown.
The most common form of pick-off occurs when the pitcher throws to a base (most commonly first base) from which the baserunner has taken a lead. The runner must return to the base before a tag is applied by the fielder receiving the throw. If he fails to do so, he is out, with the putout credited to the infielder and an assist to the pitcher. Alternatively, the runner may decide to try to run to the next base - if it is unoccupied - instead of returning to the base he is occupying. If he is successful in reaching it before the ball can be relayed there, he is credited with a stolen base; if not, he is out on a caught stealing with the pitcher receiving an assist. Most balks called are the result of improperly executed pick-off attempts.
There has long been concern that unsuccessful pick-up attempts can cause unnecessary delays and are of little interest to fans or anyone else. In 2021, MLB experimented with a couple of rule changes in the minor leagues in order to limit useless pick-off attempts: in High-A, pitchers were required to step off the mound before throwing to a base, making it a lot harder to trick a baserunner, especially for a left-handed pitcher; and in Low-A a limit of two attempts per plate appearance was installed; any further unsuccessful throw to a base results in a balk being called. The latter version was adopted at the major league level as part of the changes to the rules changes introduced for the 2023 season.
The second form of pick-off occurs when, after catching the pitch, the catcher notices that a baserunner has ventured far from his base. If his throw to that base arrives in time for a fielder to tag the baserunner before he can return to the bag, the runner is out on a pick-off.
A pick-off is considered to be a baserunning mistake, even if it not usually captured in individual statistics as it is not a caught stealing. Inversely, it is also a clutch play for the defense, which can both record an out and erase a baserunner in one stroke, without allowing any other runners to advance.