noun \ˈsī-nəs\

Definition of SINUS

: a cavity or hollow in the body: as a : a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus <a tuberculous sinus> b (1) : a cavity in the substance of a bone of the skull that usually communicates with the nostrils and contains air (2) : a channel for venous blood (3) : a dilatation in a bodily canal or vessel

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