A pickoff in baseball is successful throw from a pitcher or catcher to a fielder, usually a baseman, who then tags a base-runner attempting to steal out before he can return to the base and be called safe.
The pitcher may have the option to get a runner out by quickly throwing the baseball to a baseman for a tag out. A pickoff describes a situation in baseball where the pitcher will quickly throw the baseball to a baseman in hopes of catching the base runner off-base for a tag out.
Pickoffs most often occur at first base. Before a pitch, the pitcher will quickly throw the baseball to the first baseman, who tries to tag out the base runner with the baseball. If the first baseman tags the runner before the runner touches first base, the runner is out and the pickoff is successful.