maxillary sinus vs sinus

maxillary sinus and sinus both are nouns.

Word	Noun	Adjective	Verb	Adverb
maxillary sinus	Yes	No	No	No
sinus	Yes	No	No	No

As nouns, sinus is a hypernym of maxillary sinus; that is, sinus is a word with a broader meaning than maxillary sinus:

maxillary sinus: one of a pair of sinuses forming a cavity in the maxilla
sinus: any of various air-filled cavities especially in the bones of the skull

maxillary sinus (noun)	sinus (noun)
one of a pair of sinuses forming a cavity in the maxilla	a wide channel containing blood; does not have the coating of an ordinary blood vessel
	any of various air-filled cavities especially in the bones of the skull
	an abnormal passage leading from a suppurating cavity to the body surface