What is "levirate"

Levirate \Lev"i*rate\ (l[e^]v"[i^]*r[asl]t), Leviratical \Lev`i*rat"ic*al\ (-r[a^]t"[i^]*kal), a. [L. levir a husband's brother, brother-in-law; akin to Gr. dah`r: cf. F. l['e]virat leviration.] Of, pertaining to, or in accordance with, a law of the ancient Israelites and other tribes and races, according to which a woman, whose husband died without issue, was married to the husband's brother.

The firstborn son of a leviratical marriage was reckoned and registered as the son of the deceased brother.
--Alford.

custom by which the male next-of-kin of a dead man was bound to marry his widow, 1725, from Latin levir "brother-in-law" (from PIE *daiwer- "husband's brother") + -ate (2).

a. Having to do with one's husband's brother. n. 1 (context countable English) A marriage between a widow and her deceased husband's brother or, sometimes, heir. 2 (context anthropology English) The institution of levirate marriage.

n. the biblical institution whereby a man must marry the widow of his childless brother in order to maintain the brother's line