Sinus

Part of speech: noun (n.)

1. An opening; a hollow; a bending.

2. A bay of the sea; a recess in the shore.

3. A cavity; a depression. Specifically:

4. A cavity in a bone or other part, either closed or with a narrow opening.

5. A dilated vessel or canal.

6. A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice.

7. A depression between adjoining lobes.