Avunculus Definition: Latin: a mother's brother. Roman law developed and maintained complex rules for the passage of an estate, including priorities for step-children.For this, within a legal system that favoured male lineage, it was necessary to distinguish between relatives on the father's side and those on the mother's side.Each possible participant in a decedent's family tree was assigned a distinct name for the purposes of the hierarchy of estate distribution.William Blackstone's Commentaries on the Laws of England, Book 2, page 230:"... avunculus or mother's brother."Henry de Bracton's Legibus also referred to avunculus.Avuncular refers to an uncle.The relationship of a mother's sister to an individual is known as matertera. Categories & Topics: Dictionary of Latin Law Terms Trusts, Wills, Estates and Probate Law Dictionary Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!