A clause in a contract, or agreement, which has for its object to suspend, to defeat, or in some way to modify, the principal obligation; or, in case of a will, to suspend, revoke, or modify a devise or bequest. It is also the case of a future uncertain event, which may or may not happen, and on the occurrence or non-occurrence of which, the accomplishment, recission, or modification of an obligation or testamentary disposition is made to depend.