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)