An ethnicity or ethnic group is a grouping of people who identify with each other on the basis of shared attributes that distinguish them from other groups. Those attributes can include common sets of traditions, ancestry, language, history, society, nation, religion, or social treatment within their residing area. The term ethnicity is often used interchangeably with the term nation, particularly in cases of ethnic nationalism.

Ethnicity may be construed as an inherited or as a societally imposed construct. Ethnic membership tends to be defined by a shared cultural heritage, ancestry, origin myth, history, homeland, language, dialect, religion, mythology, folklore, ritual, cuisine, dressing style, art, or physical appearance. Ethnic groups may share a narrow or broad spectrum of genetic ancestry, depending on group identification, with many groups having mixed genetic ancestry. Ethnic groups often continue to speak related languages.

By way of language shift, acculturation, adoption and religious conversion, individuals or groups may over time shift from one ethnic group to another. Ethnic groups may be divided into subgroups or tribes, which over time may become separate ethnic groups themselves due to endogamy or physical isolation from the parent group. Conversely, formerly separate ethnicities can merge to form a pan-ethnicity and may eventually merge into one single ethnicity. Whether through division or amalgamation, the formation of a separate ethnic identity is referred to as ethnogenesis.

Although both organic and performative criteria characterise ethnic groups, debate in the past had dichotomised between primordialism and constructivism. Earlier 20th-century "Primordialists" viewed ethnic groups as real phenomena whose distinct characteristics have endured since the distant past. Perspectives that developed after the 1960s increasingly viewed ethnic groups as social constructs, with identity assigned by societal rules.