What is Determinable Freeholds?

Legal Definition
Estates for life, which may determine upon future contingencies before the life for which they are created expires. As if an estate be granted to a woman during her widowhood, or to a man until he Jbe promoted to a benefice; in these and similar cases, whenever the contingency happens, when the widow marries, or when the grantee obtains the benefice, the respective estates are absolutely determined and gone. Yet, while they subsist, they are reckoned estates for life; because they may by possibility last for life, if the contingencies upon which they are to determine do not sooner happen. 2 Bl. Comm. 121.
-- Black's Law Dictionary