custom enjoining marriage outside one's own group. In some cases, the rules of exogamy may also specify the outside group into which an individual must marry. The severity of enforcement of exogamous restrictions varies greatly across cultures and may range from death to mild disapproval. Mandatory marriage within one's own group is known as endogamy.

Exogamy is usually defined through kinship rather than ethnicity (ethnic group), religion, or class (social class). It is most common among groups that reckon descent through either the father (patrilineality) or the mother (matrilineality) alone. Such lineages (lineage) may in turn be grouped into clans (clan) or moieties (moiety system). These are most often the locus of exogamy; marrying a member of one's own clan or moiety typically constitutes a form of incest.

Exogamy does not guarantee that spouses have no genetic relationship. Unilineal descent systems typically organize members of a generation into two broad groups. Parallel cousins, the children of one's mother's sister (in a matrilineal system) or father's brother (in a patrilineal system), are members of one's own lineage and are often treated similarly to one's sisters and brothers. Cross-cousins (cross-cousin), the children of one's mother's brother (in a matrilineal system) or father's sister (in a patrilineal system), belong to a different lineage from one's own. In many exogamous cultures, cross-cousins are viewed as ideal marriage partners.