A condition in a contract that causes the contract to become invalid if a certain event occurs. A condition in a contract or will that causes the contract to become invalid if a certain event occurs.This is different from a condition precedent.The happening of a condition subsequent may invalidate a contract which is, until that moment, fully valid and binding. In the case of a condition precedent, no binding contract exists until the condition occurs. condition in a contract that causes the contract to become invalid if a certain event occurs. A condition in a contract or will that causes the contract to become invalid if a certain event occurs.