From Latin levir, "husband's brother." The biblical institution commanding that when a married man died childless, his surviving brother was to marry the widow and raise the first son to bear the dead brother's name, preserving the dead brother's line and inheritance in Israel (Deuteronomy 25:5-10). If the brother refused, the widow could publicly shame him by removing his sandal in front of the elders — "the house of him who had his sandal pulled off." The practice predates Moses (Genesis 38, Judah and Tamar) and continues in the background of Ruth (chapter 4, Boaz as kinsman-redeemer).
Levirate marriage preserves three biblical priorities. (1) Family line continuity. In an agricultural society where land was inherited by tribe and clan, a man dying childless meant his family line — and his portion of the promised land — died with him. Levirate marriage ensured that every man's name and inheritance continued, even in death. This is why the institution is so emotionally charged in the stories (Judah-Tamar, Boaz-Ruth). (2) Care for widows. A childless widow in the ancient world was extremely vulnerable — no son to protect her, no inheritance, no social safety net. Levirate marriage placed responsibility on the dead husband's extended family: his brother must step in to provide a home, a future, and children to care for her in her old age. (3) Typological significance. The book of Ruth transforms levirate marriage into one of the great pictures of the kinsman-redeemer (go'el). Boaz redeems Ruth and Naomi, buys back the land, raises up a seed to the dead (Mahlon), and produces a genealogy that includes David and eventually Jesus. Christ is the ultimate kinsman-redeemer — who entered our humanity (becoming near-of-kin), paid the redemption price (His own blood), and raises up for us an inheritance we forfeited through death. The Sadducees tried to use levirate marriage to disprove the resurrection (Matthew 22:23-33); Jesus used their question to prove it, exposing their ignorance of both Scripture and God's power.