rule against perpetuities

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   Any rule that prevents a testator or other transferor of property from controlling further transfer of his property beyond a certain length of time.