sinus vs ethmoid sinus

sinus and ethmoid sinus both are nouns.

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

As nouns, ethmoid sinus is a hyponym of sinus; that is, ethmoid sinus is a word with a more specific, narrower meaning than sinus:

sinus: any of various air-filled cavities especially in the bones of the skull
ethmoid sinus: a sinus of the meatuses of the nasal cavity (behind the bridge of the nose)

sinus (noun)	ethmoid sinus (noun)
a wide channel containing blood; does not have the coating of an ordinary blood vessel	a sinus of the meatuses of the nasal cavity (behind the bridge of the nose)
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