daydreamer vs dreamer

daydreamer and dreamer both are nouns.

Word	Noun	Adjective	Verb	Adverb
daydreamer	Yes	No	No	No
dreamer	Yes	No	No	No

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

Other hyponyms of daydreamer include lotus-eater, stargazer, escapist, wishful thinker.

daydreamer (noun)	dreamer (noun)
someone who indulges in idle or absentminded daydreaming	a person who escapes into a world of fantasy
	someone guided more by ideals than by practical considerations
	someone who is dreaming