rhombohedron vs hexahedron

rhombohedron and hexahedron both are nouns.

Word	Noun	Adjective	Verb	Adverb
rhombohedron	Yes	No	No	No
hexahedron	Yes	No	No	No

As nouns, hexahedron is a hypernym of rhombohedron; that is, hexahedron is a word with a broader meaning than rhombohedron:

rhombohedron: a parallelepiped bounded by six similar faces (either rhombuses or parallelograms)
hexahedron: any polyhedron having six plane faces

Other hypernyms of rhombohedron include parallelepiped, parallelepipedon, parallelopiped, parallelopipedon.

rhombohedron (noun)	hexahedron (noun)
a parallelepiped bounded by six similar faces (either rhombuses or parallelograms)	any polyhedron having six plane faces