An agreement made before marriage to settle on the wife upon the husband’s death some amount of money or property. In the world of Jane Austen, this was important because upon marriage, all the wife’s assets go to the husband and upon his death, the eldest son. So a jointure is important for a wife to secure, as Mrs. Dashwood discovers in Sense and Sensibility.
“Mrs. Jennings was a widow, with an ample jointure. She had only two daughters, both of whom she had lived to see respectably married, and she had now therefore nothing to do but to marry all the rest of the world.”