Noun
the relationship between a man and his sister's children, generally her sons.
(In certain tribal peoples, for example, the Korowai) a special relationship between a man and a maternal uncle, and the mutual responsibilities between them.
Source: en.wiktionary.orgSee Article 809 of the Korean Civil Code and citation An Avunculate marriage is a marriage that occurs between an uncle and his niece or between an aunt and her nephew. Source: Internet