What does "sinus" mean?

Additional senses

1. 2.(Anat. & Zoöl.) 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.
2. 3.(Med.) A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice.
3. 4.(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. (Zoöl.) See under Pallial. -- Sinus venosus. Etym: [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.