sigmoid sinus vs venous sinus

sigmoid sinus and venous sinus both are nouns.

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

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

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

Other hypernyms of sigmoid sinus include sinus.

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