N.Y. Est. Powers & Trusts Law § 6-4.9

§ 6-4.9 Definition of a future estate vested subject to complete\n defeasance\n A future estate vested subject to complete defeasance is an estate\ncreated in favor of one or more ascertained persons in being, which\nwould become an estate in possession upon the expiration of the\npreceding estates, but may end or may be terminated as provided by the\ncreator at, before or after the expiration of such preceding estates.\n