ylidEnglishOxford New American Dictionaryylid /ˈilid , ˈɪlɪd / (also ylide /ˈilīd/) ▸ noun Chemistry a compound that has an uncharged molecule containing a negatively charged carbon atom directly bonded to a positively charged atom of sulfur, phosphorus, nitrogen, or another element: ammonium ylids. – DERIVATIVES ylidic – ORIGIN 1950s : from -yl + -ide.