Definition from Nolo’s Plain-English Law Dictionary
A provision in a deed or will that attempts to restrict the sale or transfer of the property forever or for an extremely long period of time -- for example, selling your house to your daughter with the provision that it never be sold to anyone outside the family. These provisions are usually unenforceable on the grounds that a present owner should not be allowed to tie the hands of future generations. The maximum period of time for limiting transfer is generally "lives in being, plus 21 years." (This is known as the rule against perpetuities.) Restraints on alienation (restrictive covenants) based on race ("only Caucasians may hold title") were declared unconstitutional in 1949.