What is "pur autre vie"

In property law of the United States and some Canadian provinces, pur autre vie ( Old French for "for another['s] life") is a duration of a property interest. While it is similar to a life estate, it differs in that a person's life interest will last for the life of another person instead of their own. For example, if Bob is given use of the family house for as long as his mother lives, he has possession of the house pur autre vie. Pur autre vie can occur when a contingent remainder is destroyed, in a Doctrine of Merger situation, where one person acquires the life estate of another and thereby destroys a remainder not already vested.