ER

ER is an acronym which refers to:

• In cell biology, the endoplasmic reticulum, which is either rough or smooth
• the "emergency room" in a hospital, wherein victims arrive in emergency situations (usually by ambulance).
• a popular NBC medical drama, named ER for emergency room
• Eritrea (ISO country code)
• Elizabeth Regina (Elizabeth the Queen); see Elizabeth I of England and Elizabeth II of the United Kingdom.