The avunculate (sometimes called avunculism or avuncularism) is a feature of some societies whereby men have a special role in relation to their sisters' children.
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, the 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 1989 Oxford English Dictionary defines 'avunculate' as follows:"Avunculate. The special relationship existing in some societies between a maternal uncle and his sister's son; maternal uncles regarded as a collective body. 1920 R. H. LOWIE Prim. Soc. v. 81 Ethnologists describe under the heading of avunculate the customs regulating in an altogether special way the relations of a nephew to his maternal uncle. Ibid. vii. 171 The Omaha are patrilineal now, but their having the avunculate proves that they once traced descent through the mother, for on no other hypothesis can such a usage be explained. .. "