"Error" describes an event in baseball in which the official scorer judges that the fielder (defensive player) incorrectly played the ball or made a mistake that allowed for a runner to proceed to an additional base that he/she wouldn't have reached if the error hadn't been made. Errors can occur even if a runner didn't gain a base-for example, in the event of a misplayed foul ball.
If a batter gets onto base as the result of an error, the error will be recorded for the fielding player, but the batter won't receive a hit unless it is deemed likely that the batter would've reached first base without the error occurring.
This term can be used to refer to the statistic itself or to the play in which the error occurred.
The error metric is thought to give some indication as to the fielding effectiveness of a defensive player and is used in the calculation of statistics such as fielding percentage. Critics of the metric say it tends to undercount the errors a fielder makes as it often doesn't consider a lot of the mistakes made by a fielder, such as coverage errors.