Earned Run Average
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Earned run average (ERA) estimates how many earned runs a pitcher allows per 9 innings pitched, standardizing performance to a full game regardless of how many innings the pitcher actually threw. It's calculated as (earned runs / innings pitched) × 9.

An earned run is one that scores without the help of a fielding error or passed ball, meaning it's attributed to the pitcher's own performance. Unearned runs, which result from defensive mistakes, are excluded from ERA specifically to isolate the pitcher's own contribution to runs allowed.

Since a standard game is 9 innings, scaling to that number lets you compare a reliever who threw 2 innings to a starter who threw 7 innings on a level, per-game basis. Without that scaling, pitchers with different workloads couldn't be fairly compared.

In modern MLB, an ERA below 3.00 is considered excellent, around 4.00 is roughly league average, and anything above 5.00 is generally considered poor. Historical eras with different offensive levels (like the high-scoring late 1990s) shift what counts as 'good' relative to league context.

Because ERA is calculated per 9 innings, a small number of actual innings pitched can produce wild swings from a single bad outing, since even a couple of earned runs get scaled up dramatically. This is why ERA is more meaningful over a full season than over a handful of appearances.