Earned Run Average
Earned runs a pitcher allows per nine innings.
What it actually is. Earned runs allowed per nine innings. Earned means the run would have scored with clean fielding behind him, so errors and passed balls are stripped out before the arithmetic starts.
What a scout reads off it. It is the number the game has judged pitchers by for a century and it is a measure of results rather than of pitching. Read it against the pitcher's strikeouts, walks and home runs, because when those disagree with the earned run average, they are usually right and it is usually the one that moves.
Where the lines are. Under 3.00 is a very good season in most eras and under 2.00 is a historic one. Bob Gibson's 1.12 in 1968 was so far outside normal that baseball lowered the mound the following winter.
How it lies to you. The earned run rule protects a pitcher from his fielders' mistakes but not from their range, which is the larger effect by far. A ball a better shortstop reaches is a hit, and every run behind it is earned.
At Little League and travel ball. At youth levels the split between earned and unearned is close to meaningless, because a large share of runs involve a misplay somewhere. Youth leagues that track it usually compute per six or seven innings rather than nine, so the number is not comparable to a professional one without knowing which.
Batter's Eye: look up anything in baseball