Si·nus n.; pl. L. Sinus, E. Sinuses
 1. An opening; a hollow; a bending.
 2. A bay of the sea; a recess in the shore.
 3. Anat. & Zool. 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. Zool. 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.
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