What is the meaning of fee simple defeasible?

fee simple defeasible is an estate in land in which the holder has a fee simple title subject to being divested upon the happening of a specified condition; also called a qualified fee or a defeasible fee. There are two categories of fee defeasible estates-fee simple determinable and fee simple subject to a condition subsequent. The term fee simple determinable implies that the duration of the estate can be determined from the deed itself. This is not true of a fee simple subject to a condition subsequent, in which case the estate's duration depends on the grantor's independent choice of whether to terminate the estate. (See fee simple determinable, fee simple subject to a condition subsequent).