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.