What is Possibility?

Legal Definition
An uncertain thing which may happen; Lilly's Reg. h. t.; or it is a contingent interest in real or personal estate. 1 Mad. Ch. 549.

2. Possibilities are near as when an estate is limited to one after the death of another; or remote, as that one man shall be married to a woman, and then that she shall die, and he be married to another. 1 Fonb. Eq. 212, n. e; l6 Vin. Ab. h. t., p. 460; 2 Co. 51 a.

3. Possibilities are also divided into, 1. A possibility coupled with an interest. This may, of course, be sold, assigned, transmitted or devised; such a possibility occurs in executory devises, and in contingent, springing or executory uses.

4. - 2. A bare possibility, or hope of succession; this is the case of an heir apparent, during the life of his ancestor. It is evident that he has no right which he can assign, devise, or even, release.

5. - 3. A possibility' or mere contingent interest, as a devise to Paul if he survive Peter. Dane's Ab. c. 1, a 5, 2, and the cases there cited.
-- Bouviers Law Dictionary
Legal Definition
An uncertain thing which may happen. A contingent interest in real or personal estate. Kinzie v. Winston, 14 Fed. Cas. 651; Rodenhamer v. Welch, 89 N. C. 78; Needles v. Needles, 7 Ohio St. 442, 70 Am. Dec. 85. It is either near, (or ordinary,) as where an estate is limited to one after the death of another, or remote, (or extraordinary,) as where it is limited to a man, provided he marries a certain woman, and that she shall die and he shall marry another.
See also
-- Black's Law Dictionary
Legal Definition
That kind of contingent benefit which is neither the object of a limitation, like an executory interest, nor is founded in any lost but recoverable seisin, like a right of entry. See 56 Am. St. Rep. 340, note.
-- Ballentine's Law Dictionary