N.Y. Est. Powers & Trusts Law § 6-4.8

§ 6-4.8 Definition of a future estate vested subject to open\n A future estate vested subject to open is an estate created in favor\nof a class of persons, one or more of whom are ascertained and in being,\nwhich is certain when created to become an estate in possession whenever\nand however the preceding estates end, and is subject to diminution by\nreason of another person becoming entitled to share therein.\n