Rule against perpetuities

The rule against perpetuities is a legal rule in common law that prevents people from using legal instruments (usually a deed or a will) to exert control over the ownership of private property for a time long beyond the lives of people living at the time the instrument was written. Specifically, the rule forbids a person from creating future interests (traditionally contingent remainders and executory interests) in property that might feasibly vest beyond 21 years after the lifetimes of those living at the time of creation of the interest, often expressed as a "life in being plus twenty-one years".

Source: Wikipedia — Rule against perpetuities (CC BY-SA 4.0)