si·nus 
/ˈsaɪnəs/ noun
plural si·nus·es /ˈsaɪnəs/ noun [count] : any of several spaces in the skull that connect with the nostrils — usually plural ▪ My sinuses are blocked. [=they are filled with mucus that will not drain properly] — often used before another noun ▪ a sinus infection/problem
