castor oil UK /ˈkɑː.stə ˌɔɪl/US /ˈkæ.stɚ ˌɔɪl/ noun1 •The pale yellow vegetable oil extracted from the castor bean; used as a laxative and an industrial lubricant.Every six months, spoonfuls of castor oil were administered to combat constipation. — 2020 formscastor oils · castor-oils