sinussi·nus (s??n?s)n.1. A depression or cavity formed by a bending or curving.2. Anatomy a. A dilated channel or receptacle containing chiefly venous blood.b. Any of various air-filled cavities in the bones of the skull, especially one communicating with the nostrils.3. Medicine A fistula leading from a pus-filled cavity.4. Botany A recess or indentation between two lobes or segments, as of a leaf.[Middle English, hollow in the body, from Medieval Latin, from Latin, curve, hollow.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.sinus (?sa?n?s) n, pl -nuses1. (Anatomy) anatomy a. any bodily cavity or hollow spaceb. a large channel for venous blood, esp between the brain and the skullc. any of the air cavities in the cranial bones2. (Pathology) pathol a passage leading to a cavity containing pus3. (Botany) botany a small rounded notch between two lobes of a leaf, petal, etc4. (Zoology) an irregularly shaped cavity[C16: from Latin: a curve, bay]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014si?nus (?sa? n?s) n., pl. -nus?es. 1. a curve; bend. 2. a curving part or recess. 3. a. any of various cavities, recesses, or passages in the body, as a hollow in a bone or a reservoir or channel for venous blood. b. one of the hollow cavities in the skull connecting with the nasal cavities. c. an expanded area in a canal or tube. 4. a narrow passage leading to an abscess or the like. 5. a small, rounded depression between two projecting lobes, as of a leaf. [1590?1600;