n. [ a trademark. ] a brand of sweetened fruit-flavored powdered mixture, packaged so that it can be conveniently dissolved in hot water and solidified into a gelatin, used as a dessert, sometimes with pieces of fruit mixed in; also the gelatin dessert thus prepared; sometimes used metaphorically; as, when he pulled a gun on me, my knees turned to Jell-O.. [ Trademark ] Jell-O was originally a trademark and is still protected by trademark law, but is often used as one word, uncapitalized in informal writing. [ WordNet 1.5 +PJC ]