Earned Run Average (ERA)
$$ {\mathrm {ERA}}=9\times {\frac {{\mathrm {Earned~Runs~Allowed}}}{{\mathrm {Innings~Pitched}}}} $$
Number of earned runs allowed divided by the number of innings pitched, multiplied by nine. ERA is the most commonly accepted statistical tool for evaluating pitchers.
ERA is the mean of earned runs given up by a pitcher per nine innings pitched (i.e. the traditional length of a game). Runs resulting from defensive errors (including pitchers' defensive errors) are recorded as unearned runs and omitted from ERA calculations. If a pitcher exits a game with runners on base, any earned runs scored by those runners will count against him.
Factoids

The lowest career ERA is 1.82, set by Chicago White Sox pitcher Ed Walsh.

In the liveball era (post1920) Mariano Rivera holds the lowest career Major League ERA of 2.21.

Dodgers Clayton Kershaw leads all active MLB pitchers with a 2.36 career ERA.