underlayment vs carpet pad

underlayment and carpet pad both are nouns.

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

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

Other synonyms of underlayment include rug pad, underlay.

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