SI'NUS, n. [L. a bay.]

1. A bay of the sea; a recess in the shore , or an opening in the land.

2. In anatomy, a cavity in a bone or other part, wider at the bottom than at the entrance.

3. In surgery, a little cavity or sack in which pus is collected; an abscess with only a small orifice.

4. An opening; a hollow.