In baseball statistics, earned run average (ERA) is the average of earned runs allowed by a pitcher per nine innings pitched. It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Thus, a lower ERA is better. Runs resulting from passed balls, defensive errors, and runners placed on base at the start of extra innings are recorded as unearned runs and omitted from ERA calculations.

In baseball and softball statistics, an error is an act, in the judgment of the official scorer, of a fielder misplaying a ball in a manner that allows a batter or baserunner to advance one or more bases or allows a plate appearance to continue after the batter should have been put out. The term error is sometimes used to refer to the play during which an error was committed.