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 emphasizing homogeneity or racial purity as a key membership criterion).
Ethnicity is often used synonymously with terms such as nation or people. In English, it can also have the connotation of something exotic (cf. "ethnic restaurant", etc.), generally related to cultures of more recent immigrants, who arrived after the dominant population of an area was established.
The largest ethnic groups in modern times comprise hundreds of millions of individuals (Han Chinese being the largest), while the smallest are limited to a few dozen individuals (numerous indigenous peoples worldwide). 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. 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.
The term ethnic is derived from the Greek word ἔθνος ethnos (more precisely, from the adjective ἐθνικός ethnikos, which was loaned into Latin as ethnicus). The inherited English language term for this concept is folk, used alongside the latinate people since the late Middle English period.
In Early Modern English and until the mid-19th century, ethnic was used to mean heathen or pagan (in the sense of disparate "nations" which did not yet participate in the Christian oikumene), as the Septuagint used ta ethne ("the nations") to translate the Hebrew goyim "the nations, non-Hebrews, non-Jews". The Greek term in early antiquity (Homeric Greek) could refer to any large group, a host of men, a band of comrades as well as a swarm or flock of animals. In Classical Greek, the term took on a meaning comparable to the concept now expressed by "ethnic group", mostly translated as "nation, people"; only in Hellenistic Greek did the term tend to become further narrowed to refer to "foreign" or "barbarous" nations in particular (whence the later meaning "heathen, pagan").
In the 19th century, the term came to be used in the sense of "peculiar to a race, people or nation", in a return to the original Greek meaning. The sense of "different cultural groups", and in American English "racial, cultural or national minority group" arises in the 1930s to 1940s, serving as a replacement of the term race which had earlier taken this sense but was now becoming deprecated due to its association with ideological racism. The abstract ethnicity had been used for "paganism" in the 18th century, but now came to express the meaning of an "ethnic character" (first recorded 1953). The term ethnic group was first recorded in 1935 and entered the Oxford English Dictionary in 1972. Depending on the context that is used, the term nationality may either be used synonymously with ethnicity, or synonymously with citizenship (in a sovereign state). The process that results in the emergence of an ethnicity is called ethnogenesis, a term in use in ethnological literature since about 1950.
Depending on which source of group identity is emphasized to define membership, the following types of (often mutually overlapping) groups can be identified: