ER or Er may refer to:

• Elizabeth Regina, the Latin form of Queen Elizabeth
• Eduardus Rex, the Latin form of King Edward
• Eleanor Roosevelt, former First Lady of the United States
• Er (biblical person), the eldest son of Judah in the biblical book of Genesis
• Er, a character in Plato's Myth of Er
• Èr, an ancient people inhabiting parts of what are now northern Armenia, northeast Turkey, southern Georgia, and northwest Azerbaijan
• Nie Er (1912–1935), Chinese composer
• E. R. Eswaran (born 1961), Indian politician and entrepreneur from Tamil Nadu

.er is the country code top-level domain (ccTLD) for Eritrea.

In English, the -er suffix can signify:

• an agent noun, e.g., "singer"
• a degree of comparison, e.g., "noisier"
• Oxford "-er", a colloquial and sometimes facetious suffix prevalent at Oxford University from about 1875

A hormone-receptor-positive tumor is a tumor which consists of cells that express receptors for certain hormones. The term most commonly refers to estrogen receptor positive tumors (i.e. tumors that contain estrogen receptor positive cells), but can also include progesterone receptor positive tumors. Estrogen-receptor-positive tumors depend on the presence of estrogen for ongoing proliferation.

The Èr people, also known as Èrsh or (in Georgian works) the Hers, are a little-known ancient people inhabiting northern modern Armenia, and to an extent, small areas of northeast Turkey, southern Georgia, and northwest Azerbaijan. Most of their history is constructed based on archaeological and linguistic (primarily based on placenames, with some elements) data, compared to historical trends in the region and historical writings, such as the Georgian Chronicles or the Armenian Chronicles, as well as a couple notes made by Strabo. They were a constituent of the state of Urartu, which either incorporated or conquered them during the 8th century BCE. Their relation to the main Urartians (who were probably ethnically separate from them, judging from place names) is unknown. Linguistically, based on placenames, they are thought to have been a Nakh people.