insecticide UK /ɪnˈsɛk.tɪˌsaɪd/US /ɪnˈsɛk.təˌsaɪd/ noun1 •Any of various substances used to kill insects.The dried seed powder of custard apple (Annona squamosa L; family: Annonaceae) has been used as an insecticide in India and many other tropical countries since ancient times. — 2009 formsinsecticides