The term "PO" in baseball refers to "Put Out." The play made by a fielder against a batter or baserunner that results in an out being recorded is called a putout. This can be stopping a baserunner in their tracks, catching a batted ball in the air, or making a play to retire a baserunner. Putouts are attributed to specific fielders according to their role in recording outs during a game and are one of the main defensive metrics monitored in baseball.