The term avunculate is a technical term used in kinship anthropology to describe relationship between someone and his/her maternal uncle, or, on the other side, between the person and his/her uterine nephew or niece.
The term "avunculate" comes from the Latin word "Avunculus", which is a kinship term used to describe the brother of the mother, in opposition to the brother of the father "Patruus". In the societies where maternal filiation is strongly represented, the role of a father could be taken over by a maternal uncle, who would become a "social father" of his sister's children.
The first literary message about avunculate belongs to Tacitus (Germania, VIII, 18-20).
The notion of "avunculate" does not only pertain to the line of descent (mostly rare), but can also describe an expression of matrimonial alliance. The marriage of a man with the daughter of his sister is also called "avunculate" marriage. The term "avunculate" applies to what ties the brother of the mother and the son/daughter of the sister, and it also can be used to describe the relationship between a paternal uncle and a child of his brother .
At avunculate time the maternal family is already headed by a man, a father or a brother of the woman given into another's family or clan. The matrilocal spousal residence is replaced with patrilocal one, the man does not move any more into the house of his wife, but just the opposite, in marriage he takes her into his house. At the same time a wife and her children retain their affiliation with the former maternal family and clan. In such system the factual father of the child, instead of the blood father, is the uncle on the maternal side. And while a mother remains in the husband's house, her children (sons) "return home". The blood father and his relatives are obligated to turn the child over to his uncle, "return" him to his family. M.O.Kosven called this order "return of the children". The nephews are all-powerful and have exclusive privileges in the family of the uncle on the maternal side.
Relics of avunculate custom are known at present. According to the Kazakh common law, the avunculate nephews could take anything from the relatives of the mother up to three times. In the Kyrgyz past a nephew, at a feast at his maternal uncle or grandfather, could take any horse from their herd or any delicacy.