In baseball, an error is the act, in the judgment of the official scorer, of a fielder misplaying a ball in a manner that allows a batter or runner to reach one or more additional bases, on a play that would normally be completed successfully with ordinary effort. It normally does not count as a hit and the batter will not be credited for any RBIs when runs are scored—unless, in the scorer's judgment, the batter would have reached first base safely but one or more of the additional base(s) reached was the result of the fielder's mistake. In that case, the play will be scored both as a hit (for whatever number of bases the fielders should have limited the batter to) and an error. An error is also credited to a fielder who drops a foul fly and prolongs the batter's at-bat, whether or not that batter later reaches base.

A passed ball or a wild pitch is not considered an error.

Arguably, one weakness of the error as a statistic is that mental misjudgments, such as failure to cover a base, are not considered errors.

A more serious objection to the error, at least among sabermetricians, is more conceptual. In order for a fielder to be charged with an error, he must have done something right by being in the correct place to make a play. A poor fielder may "avoid" many errors simply by being unable to reach many balls that a better fielder can successfully reach, possibly creating a situation where the poor fielder will have fewer errors than an otherwise better fielder. In recent times, official scorers have recognized this; they will usually take a fielder's supposed "extraordinary" effort or positioning into account when judging whether the play should have been successful given ordinary effort.