Sinus \Si"nus\, n.; pl. L. Sinus, E. Sinuses. [L., a bent surface, a curve, the folds or bosom of a garment, etc., a bay. Cf. Sine, n.]

1. An opening; a hollow; a bending.

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

3. (Anat. & Zo["o]l.) A cavity; a depression. Specifically:
   (a) A cavity in a bone or other part, either closed or with a narrow opening.
   (b) A dilated vessel or canal.

4. (Med.) A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice.

5. (Bot.) A depression between adjoining lobes.

Note: A sinus may be rounded, as in the leaf of the white oak, or acute, as in that of the red maple.

Pallial sinus. (Zo["o]l.) See under Pallial.

Sinus venosus. [L., venous dilatation.]

(Anat.)
   (a) The main part of the cavity of the right auricle of the heart in the higher vertebrates.
   (b) In the lower vertebrates, a distinct chamber of the heart formed by the union of the large systematic veins and opening into the auricle.

Source: Webster's Revised Unabridged Dictionary (1913)