Webster's English Dictionary

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sinus

si.nus \'si--n*s\ n [NL, fr. L, curve, fold, hollow] : CAVITY, HOLLOW : as  
   : a narrow elongated tract extending from a focus of suppuration and serv 
   ing for the discharge of pus : a cavity in the substance of a bone of the 
   skull that usu. communicate s with the nostrils and contains air : a 
   channel for venous blood  : a dilatation in a bodily canal or vessel  : a 
   cleft or indentation between adjoining lobes