Noun and する verb
げんめつ
彼氏が道端で立ちションしてるのを見ちゃってさ。もう、完璧に幻滅した。
I saw my boyfriend taking a piss in the street and I was completely disillusioned.
The illusion is destroyed. If your illusion is destroyed, that is disillusionment.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.