An ethnic group or ethnicity is a category of people who identify with each other based on similarities, such as common ancestral, language, social, cultural or national experiences.

Unlike other social groups (wealth, age, hobbies), ethnicity is often an inherited status based on the society in which one lives. In some cases, it can be adopted if a person moves into another society. Membership of an ethnic group tends to be defined by a shared cultural heritage, ancestry, origin myth, history, homeland, language or dialect, symbolic systems such as religion, mythology and ritual, cuisine, dressing style, art, and physical appearance.

Ethnic groups, derived from the same historical founder population, often continue to speak related languages and share a similar gene pool. By way of language shift, acculturation, adoption and religious conversion, it is sometimes possible for individuals or groups to leave one ethnic group and become part of another (except for ethnic groups emphasising racial purity as a key membership criterion).

Depending on which source of group identity is emphasized to define membership, the following types of (often mutually overlapping) groups can be identified:

Ethno-linguistic, emphasizing shared language, dialect or script — example: Gaels
Ethno-national, emphasizing a shared polity or sense of national identity — example: Iraqis
Ethno-racial, emphasizing shared physical appearance based on genetic origins — example: African Americans
Ethno-regional, emphasizing a distinct local sense of belonging stemming from relative geographic isolation — example: South Islanders
Ethno-religious, emphasizing shared affiliation with a particular religion, denomination or sect — example: Sikhs

Larger ethnic groups may be subdivided into smaller sub-groups known variously as tribes or clans, which over time may become separate ethnic groups themselves due to endogamy or physical isolation from the parent group.