underlayment vs rug pad

underlayment and rug pad both are nouns.

Word	Noun	Adjective	Verb	Adverb
underlayment	Yes	No	No	No
rug pad	Yes	No	No	No

As nouns, underlayment and rug pad are synonyms defined as:

Other synonyms of underlayment include carpet pad, underlay.

underlayment (noun)	rug pad (noun)
a pad placed under a carpet	a pad placed under a carpet