Sinus is a term used to describe the air-filled cavities in the skull. The first crossword solution for this term is 'CAVITY, ' which refers to the hollow spaces in the skull that contain air. The second solution is 'antrum, ' which is a specific type of CAVITY found in the maxillary sinus. The third solution is 'RECESS, ' which is another name for a CAVITY or hollow space. All three solutions refer to the same concept, which is the air-filled cavities in the skull known as sinuses.