Medicine

• Emergency room, a department of a hospital used for quick and needed medical attention
• Estrogen receptor, a member of the nuclear hormone family of intracellular receptor

Television

• ER (TV series), a 1994–2009 NBC medical drama
• E/R, a 1984–1985 CBS sitcom

Science and computing

• Engineer, a lawful holder of a degree in engineering
• Endoplasmic reticulum, a type of organelle in cell biology
• Estrogen receptor, a type of receptor in biochemistry
• Equine Exertional Rhabdomyolysis, a disease found in horses
• Erbium, a chemical element
• Entity-relationship model or diagram, a conceptual data model used in software engineering and database design
• Relative static permittivity, a concept in chemistry represented as $epsilon\_r$

Language

• Er (Cyrillic), a letter in the Cyrillic alphabet
• The suffix -er
• A filler word used in dialogue displaying uncertainty
• Third person nominative pronoun in German

Other meanings

• Er (Biblical name)
• ER (Extended Range), a term for missiles or aircraft
• Myth of Er, an analogy in Plato's The Republic
• Eesti Rahvusringhääling, a public broadcasting organization in Estonia
• Earned run, a concept in the game of baseball
• Evidential Reasoning Approach, a concept in Decision Theory
• United Russia, a political party
• Chico Enterprise-Record, a daily newspaper
• Eritrea, an African country (ER ISO 3166-1 alpha-2 code)
• Astar Air Cargo, an airline (ER IATA code)
• Eduardus Rex, Latin initials used by King Edward I through VII
• Elizabeth Regina, initials used by Queen Elizabeth II (only "Regina" is in Latin)