will-o'-the-wisp vs illusion

will-o'-the-wisp and illusion both are nouns.

Word	Noun	Adjective	Verb	Adverb
will-o'-the-wisp	Yes	No	No	No
illusion	Yes	No	No	No

As nouns, illusion is a hypernym of will-o'-the-wisp; that is, illusion is a word with a broader meaning than will-o'-the-wisp:

Other hypernyms of will-o'-the-wisp include fancy, fantasy, phantasy.

will-o'-the-wisp (noun)	illusion (noun)
an illusion that misleads	the act of deluding; deception by creating illusory ideas
a pale light sometimes seen at night over marshy ground	an illusory feat; considered magical by naive observers
	something many people believe that is false
	an erroneous mental representation