oneirism vs air castle

oneirism and air castle both are nouns.

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

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

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

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