What is Condition Subsequent?

Legal Definition
A condition subsequent is an event or state of affairs that brings an end to something else. A condition subsequent is often used in a legal context as a marker bringing an end to one's legal rights or duties. A condition subsequent may be either an event or a state of affairs that must either (1) occur or (2) fail to continue to occur.
-- Wikipedia
Legal Definition
A rule in a contract that allows interest to be defeated if a specific event occurs or doesn't occur. Refer to conditions concurrent and condition precedent.
Legal Definition
One which follows the principal act or obligation, often as a defeasance. See 34 N. J. L. 496, 3 Am. Rep. 256.
-- Ballentine's Law Dictionary