rug pad vs underlayment

rug pad and underlayment both are nouns.

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

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

Other synonyms of rug pad include carpet pad, underlay.

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