Pronunciation: (sī'nus),[key] —n., —pl.-nus•es. 1. a curve; bend.
2. a curving part or recess.
3. Anat. a. any of various cavities, recesses, or passages, as a hollow in a bone, or a reservoir or channel for venous blood.
b. one of the hollow cavities in the skull connecting with the nasal cavities.
c. an expanded area in a canal or tube.
4. Pathol.a narrow passage leading to an abscess or the like.
5. Bot.a small, rounded depression between two projecting lobes, as of a leaf.