illusion vs fantasm

illusion and fantasm both are nouns.

Word	Noun	Adjective	Verb	Adverb
illusion	Yes	No	No	No
fantasm	Yes	No	No	No

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

Other hyponyms of illusion include apparition, phantasm, phantasma, phantom, shadow, irradiation, phantom limb.

illusion (noun)	fantasm (noun)
the act of deluding; deception by creating illusory ideas	something existing in perception only
an illusory feat; considered magical by naive observers	a ghostly appearing figure
something many people believe that is false
an erroneous mental representation