N.Y. Est. Powers & Trusts Law § 6-4.7

§ 6-4.7 Definition of a future estate indefeasibly vested\n A future estate indefeasibly vested is an estate created in favor of\none or more ascertained persons in being which is certain when created\nto become an estate in possession whenever and however the preceding\nestates end and which can in no way be defeated or abridged.\n