SINUS

[si.nus] n [ME, fr. ML, fr. L, curve, fold, hollow] (15c): cavity, hollow: as a: a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus b (1): a cavity in the substance of a bone of the skull that usu. communicates with the nostrils and contains air (2): a channel for venous blood (3): a dilatation in a bodily canal or vessel c: a cleft or indentation between adjoining lobes (as of a leaf or corolla)