Disappointment or loss of idealism when one's expectations or illusions are shattered; disenchantment.
子供のころ憧れていた俳優のスキャンダルを知って幻滅した。
I was disillusioned when I learned about the scandal of the actor I admired as a child.
現場の厳しさを知って、入社当初の理想に幻滅した。
After learning how harsh things were on the ground, I became disillusioned with the ideals I had when I joined the company.
期待が大きすぎると、結果に対してすぐ幻滅してしまうことがある。
If expectations are too high, one can quickly become disillusioned by the outcome.