1) Air-filled extensions of the respiratory part of the nasal cavity into the frontal, ethmoid, sphenoid, and maxillary cranial bones. They vary in size and form in different individuals and are lined by the ciliated mucous membranes of the nasal cavity.

2) Any of various sinuses (as the maxillary sinus and frontal sinus) in the bones of the face and head that are lined with mucous membrane derived from and continuous with the lining of the nasal cavity.