foramen magnum vs hiatus

foramen magnum and hiatus both are nouns.

Word	Noun	Adjective	Verb	Adverb
foramen magnum	Yes	No	No	No
hiatus	Yes	No	No	No

As nouns, hiatus is a hypernym of foramen magnum; that is, hiatus is a word with a broader meaning than foramen magnum:

foramen magnum: the large opening at the base of the cranium through which the spinal cord passes
hiatus: a natural opening or perforation through a bone or a membranous structure

Other hypernyms of foramen magnum include foramen.

foramen magnum (noun)	hiatus (noun)
the large opening at the base of the cranium through which the spinal cord passes	a natural opening or perforation through a bone or a membranous structure
	a missing piece (as a gap in a manuscript)
	an interruption in the intensity or amount of something