term:

Rule Against Perpetuities

rule against perpetuities

often cap R&P
: a common-law rule stating that in order for a future interest to be good it must vest after its creation (as at the death of a testator) within a life in being or lives in being plus 21 years plus the period of gestation of any beneficiary conceived but not yet born compare life in being, statutory rule against perpetuities, wait and see

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.