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