the child of one's mother's brother or father's sister. Scholars of kinship distinguish the different types of first cousin as follows: the children of a father's siblings are patrilateral cousins, and those of a mother's siblings are matrilateral cousins; the children of a mother's sister or of a father's brother are parallel cousins (sometimes called ortho-cousins); and the children of a father's sister or of a mother's brother are cross-cousins.

Whereas the English word cousin covers all of these relationships, many languages place parallel cousins and cross-cousins in separate categories; in such cases, the terms used to denote parallel cousins are frequently the same as those denoting siblings. A smaller group of languages places the parallel cousins in one category but distinguishes the patrilateral cross-cousins from the matrilateral cross-cousins. Still another group of languages uses separate words for each of the four possible kinds of cousins: patrilateral cross-cousins, patrilateral parallel cousins, matrilateral cross-cousins, and matrilateral parallel cousins.

Some societies consider first-cousin marriages to be ideal. In those that differentiate between cross-cousins and parallel cousins, cross-cousin marriage is usually preferred or sometimes even obligatory, while marriage between parallel cousins often falls under incest taboos (taboo). In exceptional cases, however, the opposite is true. Even when a cross-cousin is the preferential mate, actual instances of such marriages in a given society may be small, for such decisions are also influenced by local customs governing succession, inheritance, and residence. Cross-cousin marriage often acts as a device to strengthen alliances between clans (clan).