What is "earned run average"

n. (context baseball English) a statistic in baseball which equals the number of earned runs allowed per nine innings pitched.

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In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched (i.e. the traditional length of a game). It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Runs resulting from defensive errors (including pitchers' defensive errors) are recorded as unearned runs and omitted from ERA calculations.