carpet pad vs underlayment

carpet pad and underlayment both are nouns.

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

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

Other synonyms of carpet pad include rug pad, underlay.

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