In the anthropological study of kinship, a moiety is a descent group that coexists with only one other descent group within a society. In such cases the community usually has unilineal descent, either patri- or matri-lineal so that any individual belongs to one of the two moiety groups by birth, and all marriages take place between members of opposite moieties. It is the same thing as an exogamous clan system with only two clans.
In the case of a patrilineal descent system this can be interpreted as a system in which women are exchanged between the two moieties. Moiety societies are found particularly among the indigenous cultures of North America and Australia.