Liable or subject to death. Of or relating to humankind; human: the mortal limits of understanding. Of, relating to, or accompanying death: mortal throes. Causing death; fatal: a mortal wound. See Synonyms at fatal. Fighting or fought to the death; unrelenting: a mortal enemy; a mortal attack. Of great intensity or severity; dire: mortal terror. Conceivable: no mortal reason for us to go. Used as an intensive: a mortal fool. A human.