Restraint on alienation is the term that describes written language in certain instruments that restricts what a recipient can do with real property. These stipulations are generally unenforceable when they are meant to ensure that the restrictions remain in place for an unreasonable amount of time.
In the United States, race-based restrictions regarding the disposal of property have been unconstitutional since 1949.
There are some circumstances under which restraint on alienation is acceptable and may be enforced.
For example, a will or deed containing language that sets property aside in order to preserve open space may be enforced. A stipulation that allows someone to buy land before anyone else has an opportunity to do so may also be enforced.
In many cases, however, restraint on alienation is frowned upon because it contradicts widely held principles pertaining to the unhindered transfer of property.