napkin vs dinner napkin

napkin and dinner napkin both are nouns.

Word	Noun	Adjective	Verb	Adverb
napkin	Yes	No	No	No
dinner napkin	Yes	No	No	No

As nouns, dinner napkin is a hyponym of napkin; that is, dinner napkin is a word with a more specific, narrower meaning than napkin:

napkin: a small piece of table linen that is used to wipe the mouth and to cover the lap in order to protect clothing
dinner napkin: a large napkin used when dinner is served

Other hyponyms of napkin include bib, tea napkin.

napkin (noun)	dinner napkin (noun)
a small piece of table linen that is used to wipe the mouth and to cover the lap in order to protect clothing	a large napkin used when dinner is served
garment consisting of a folded cloth drawn up between the legs and fastened at the waist; worn by infants to catch excrement