Earned Run
A run that would have scored without an error or passed ball.
What it actually is. A run that would have scored with ordinary fielding behind the pitcher, reconstructed by the official scorer inning by inning.
What a scout reads off it. The reconstruction is a judgment call and it is made by one person in the press box. That is worth remembering before treating the third decimal place of an earned run average as precise.
Batter's Eye: look up anything in baseball