custom or law decreeing that a widow should, or in rare cases must, marry her dead husband's brother. The term comes from the Latin levir, meaning “husband's brother.” The “brother” may be a biological sibling of the deceased or a person who is socially classified as such. Where the brother is required to be younger than the deceased, the custom is called the junior levirate. The levirate often co-occurs with the sororate, a practice in which a widower should or must marry his dead wife's sister.

Often, the brother who marries his former sister-in-law is a proxy for the deceased, in which case all progeny of the new marriage are socially acknowledged as the children of the dead man. For instance, in ancient Hebrew society, the levirate served to perpetuate the line of a man who died without offspring. Likewise, among the Nuer people of The Sudan, the children of a remarried widow belong to the first husband's line, and they consider the deceased to be their father even if the new husband is their biological genitor.