castle in the air vs oneirism

castle in the air and oneirism both are nouns.

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

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

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

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