let down vs disillusion

let down is not a noun while disillusion is a noun.

Word	Noun	Adjective	Verb	Adverb
let down	No	No	Yes	No
disillusion	Yes	No	Yes	No

As verbs, disillusion is a hyponym of let down; that is, disillusion is a word with a more specific, narrower meaning than let down:

Other hyponyms of let down include betray, fail, come short, fall short, disenchant.

let down (noun)	disillusion (noun)
	freeing from false belief or illusions

let down (verb)	disillusion (verb)
fail to meet the hopes or expectations of	free from enchantment
move something or somebody to a lower position