Definition of "ER"

Initialism of Elizabeth Regina (“Queen Elizabeth”).  examples

(British) Initialism of Edward Rex. (on coins, letterboxes, etc.) The royal cypher for Edward VII, king of Great Britain  examples

(US politics) Initialism of Eleanor Roosevelt.  examples

(US sports) Abbreviation of Erie (“Erie, Pennsylvania, USA”).  examples

(baseball) Initialism of earned run.  examples

(biology) Initialism of endoplasmic reticulum.

(computing) Initialism of entity-relationship.  examples

(medicine) Initialism of emergency room.  examples

(biochemistry) Initialism of estrogen receptor.  examples

(physics) Initialism of Einstein-Rosen bridge.  examples

(military) Initialism of enhanced radiation.