D.C. Code § 42-512: Future estates — Vested and contingent.

A future estate is vested when there is a person in being who would have an immediate right to the possession of the land upon the expiration of the intermediate or precedent estate, or upon the arrival of a certain period or event when it is to commence in possession. It is contingent when the person to whom or the event upon which it is limited to take effect in possession or become a vested estate is uncertain.