SINUS

noun Etymology: Middle English, from Medieval Latin, from Latin, curve, fold, hollow Date: 15th century cavity , hollow : as, a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus, b. a cavity in the substance of a bone of the skull that usually communicates with the nostrils and contains air, a channel for venous blood, a dilatation in a bodily canal or vessel, a cleft or indentation between adjoining lobes (as of a leaf or corolla)