malathionPronunciation: /ˌmaləˈθʌɪən/Definition of malathion noun [mass noun] a synthetic organophosphorus compound which is used as an insecticide. Origin: 1950s: from (diethyl) mal(eate) (see maleic acid) + thio- + -onmalathion in other Oxford dictionaries Definition of malathion in the US English dictionary