Sinus
SI'NUS, noun [Latin 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.