oneirism vs castle in the air

oneirism and castle in the air both are nouns.

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

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

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

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