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Also sinu‑.

A sinus.

Latin sinus, a curve, bend, or cavity.

A sinus is a cavity within a bone or other tissue; the sinoatrial node is a small group of cells in the wall of the right atrium of the heart that acts as a pacemaker; sinusitis is inflammation of the sinuses of the face. A sinusoid is either a small blood vessel in certain organs, especially the liver, or a curve having the form of a sine wave; the name of that trigonometric function comes from the large circle used to calculate its values, the sinus totus, the ‘complete curve’. Something sinuous has many curves and turns.

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