levirate
A marriage between a widow and her deceased husband's brother or, sometimes, heir.
countable
The institution of levirate marriage.
And it is, he says, impossible not to believe that the Levirate—that is, the practice of marrying a dead brother's widow—is derived from polyandry.
Where it comes from
From Latin lēvir (“husband's brother, brother-in-law”) + -ate (adjective-forming suffix), ultimately from Proto-Indo-European *dayh₂wḗr (“one's brother-in-law”).
Adapted from the Wiktionary entry levirate (CC BY-SA 4.0).