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. [ 1913 Webster ]

2. A bay of the sea; a recess in the shore. [ 1913 Webster ]

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. [ 1913 Webster ]

4. (Med.) A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice. [ 1913 Webster ]

5. (Bot.) A depression between adjoining lobes. [ 1913 Webster ]

☞ A sinus may be rounded, as in the leaf of the white oak, or acute, as in that of the red maple. [ 1913 Webster ]

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. [ 1913 Webster ]

n. [ Sinus + -oid. ] (Geom.) The curve whose ordinates are proportional to the sines of the abscissas, the equation of the curve being y = a sin x. It is also called the curve of sines. [ 1913 Webster ]

a. (Geom.) Of or pertaining to a sinusoid; like a sinusoid. [ 1913 Webster ]