An estate in lands, the existence of which depends upon the happening or not happening of some uncertain event whereby the estate may be either originally created, or enlarged, or finally defeated. 2 Bl. Comm. 151; 1 Steph. Comm. 276; Co. Litt. 201a. An estate having a qualification annexed, to it by which it may, upon the happening of a particular event be created, or enlarged, or destroyed. 4 Kent Comm. 121. Estate upon conditions expressed. An estate granted, either in fee simple or otherwise, with an express qualification annexed, whereby the estate granted shall either commence, be enlarged, or be defeated upon performance or breach of such qualification or condition. 2 BL Comm. 154. An estate which is so expressly defined and limited by the words of its creation that it cannot endure for any longer time than till the contingency happens upon which the estate is to fail. 1 Steph. Comm. 278. Estate upon condition implied. An estate having a condition annexed to it inseparably from its essence and constitution, although no condition be expressed in words. 2 Bl. Comm. 152; 4 Kent, Comm. 121.