Noun
(law, singular only) The rule that prevents a testator or other transferor of property from controlling further transfer of his property more than twenty-one years after the death of anyone alive at the time of the original transfer who may have some interest in the transfer.
(law, countable, by extension) Any rule that prevents a testator or other transferor of property from controlling further transfer of his property beyond a certain length of time.
Source: en.wiktionary.org