Rule against perpetuities
The Rule against Perpetuities is one of the most complicated rules in property law and wills and trusts. In general, the rule states that a gift of a will (or a trust) will not be enforced if the subject of the gift is to be given to a person who can not be known now, and will not be able to be known, by the end of 21 years after the death of everyone who was alive (including children in their mother's uterus but not yet born) at the time the gift was created.
For example, if a woman says in her will that her house is to be given "to the children of her children when they graduate from college" this would be not be enforced by a court. It would not be enforced because one (or more) of her children's children might not graduate from college until 21 years after everyone who was alive at the time of the woman's death has died.
The purpose of the rule is to prevent property from being tied up for "perpetuity" (a very long time). The courts (and many people in society) do not want to hold property to see who will receive it after a complicated set of rules from the gift-giver. First, this would result in a waste of the courts' time and resources. Second, it would be a waste of the property that is being given through the gift. If a gift in a will violates the Rule against Perpetuities, the court will simply strike that gift and transfer the subject of the gift as if the will had not mentioned it.
However, many countries and states have changed this rule, and it does not apply at all, or in a changed way, in those countries and states.