An estate in lands, the existence of which dependsupon the happening or not happening of some uncertain event, whereby the estatemay be either originally created, or enlarged, or finally defeated. 2 Bl. Comm. 151; 1Steph. Comm. 276; Co. Litt. 201a. An estate having a qualification annexed to it, by which it may, upon the happening of a particularevent, be created, or enlarged, or destroyed. 4 Kent, Comm. 121.