underlay vs carpet pad

underlay and carpet pad both are nouns.

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

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

Other synonyms of underlay include rug pad, underlayment.

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

underlay (verb)	carpet pad (verb)
provide with a base, support, lining, or backing
put (something) under or beneath
raise or support (the level of printing) by inserting a piece of paper or cardboard under the type