Sinus cavities produce the mucus that nasal passages need to work effectively. Read medical definition of Sinus. Define sinus. sinus synonyms, sinus pronunciation, sinus translation, English dictionary definition of sinus. sinus n. 1. An air-filled cavity in a dense portion of a skull bone.The sinuses decrease the weight of the skull. Sinus: 1. Also learn about conditions, tests, and treatments affecting the sinuses. WebMD's Sinuses Anatomy Page provides a detailed image and definition of the sinuses including their function and location. Sinusitis is a common condition defined as inflammation of the paranasal sinuses. The sinuses are formed in four right-left pairs. Medical Author: William C. Shiel Jr., MD, FACP, FACR; Tachycardia, sinus: A fast heartbeat (tachycardia) because of rapid firing of the sinoatrial (sinus) node. The sinoatrial (or sinus) node is the natural pacemaker of the heart. A depression or cavity formed by a bending or curving. Medical Definition of Tachycardia, sinus.