Main Entry: si·nus

Pronunciation: \ˈsī-nəs\

Function: noun

Etymology: Middle English, from Medieval Latin, from Latin, curve, fold, hollow

Date: 15th century

: 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 usually 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)