pelvis vs cavum

pelvis and cavum both are nouns.

Word	Noun	Adjective	Verb	Adverb
pelvis	Yes	No	No	No
cavum	Yes	No	No	No

As nouns, cavum is a hypernym of pelvis; that is, cavum is a word with a broader meaning than pelvis:

pelvis: a structure shaped like a funnel in the outlet of the kidney into which urine is discharged before passing into the ureter
cavum: (anatomy) a natural hollow or sinus within the body

Other hypernyms of pelvis include bodily cavity, cavity.

pelvis (noun)	cavum (noun)
the structure of the vertebrate skeleton supporting the lower limbs in humans and the hind limbs or corresponding parts in other vertebrates	(anatomy) a natural hollow or sinus within the body
a structure shaped like a funnel in the outlet of the kidney into which urine is discharged before passing into the ureter