Wiktionary
paranasal sinus
n. (context anatomy English) A number of air-filled spaces, or sinuses, within the bones of the skull and face.
WordNet
paranasal sinus
n. any of the paired sinuses in the bones of the face adjacent to the nasal cavity that are lined with mucous membrane that is continuous with lining of the nasal cavities [syn: sinus paranasales, nasal sinus]