sinus vs sigmoid sinus

sinus and sigmoid sinus both are nouns.

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

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

sinus: a wide channel containing blood; does not have the coating of an ordinary blood vessel
sigmoid sinus: an S-shaped dural sinus on the temporal and occipital bones

sinus (noun)	sigmoid sinus (noun)
a wide channel containing blood; does not have the coating of an ordinary blood vessel	an S-shaped dural sinus on the temporal and occipital bones
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