bas relief vs rilievo

bas relief and rilievo both are nouns.

Word	Noun	Adjective	Verb	Adverb
bas relief	Yes	No	No	No
rilievo	Yes	No	No	No

As nouns, rilievo is a hypernym of bas relief; that is, rilievo is a word with a broader meaning than bas relief:

bas relief: a sculptural relief in which forms extend only slightly from the background; no figures are undercut
rilievo: sculpture consisting of shapes carved on a surface so as to stand out from the surrounding background

Other hypernyms of bas relief include embossment, relief, relievo, sculptural relief.

bas relief (noun)	rilievo (noun)
a sculptural relief in which forms extend only slightly from the background; no figures are undercut	sculpture consisting of shapes carved on a surface so as to stand out from the surrounding background