​From the Nosebleeds - ​Going beyond the score.

What is an RBI, and how is it scored?

RBI is an acronym for Run Batted In.  Consequently, the plural form is still RBI, or Runs Batted In.  If you hear someone say RBIs, which happens a lot, you don't necessarily have to be a jerk and correct them, but you can know that it is incorrect.

A batter earns an RBI for each run that is scored as a result of that batter's base hit, sacrifice fly, sacrifice bunt, ground out, base on balls (BB), or hit-by-pitch (HBP).  If a run scores due to an defensive error or a batted ground-into-double-play (GIDP), the batter does not earn an RBI.