daydreaming vs castle in the air

daydreaming and castle in the air both are nouns.

Word	Noun	Adjective	Verb	Adverb
daydreaming	Yes	No	No	No
castle in the air	Yes	No	No	No

As nouns, daydreaming and castle in the air are synonyms defined as:

Other synonyms of daydreaming include air castle, castle in Spain, daydream, oneirism, reverie, revery.

daydreaming (noun)	castle in the air (noun)
absentminded dreaming while awake	absentminded dreaming while awake