Disillusionment is the feeling, or the slow process, of losing your belief that someone is good or that an idea is right. It is the noun that sits beside disillusioned. Essays and news like it: public disillusionment, a sense of disillusionment among young teachers. It names the state, not one sharp minute of sadness. You can feel disappointment on Tuesday and be fine on Wednesday; disillusionment tends to settle in and change how you see the whole thing. It is more formal than “I don’t believe them any more,” but it is not a rare or technical word. Though actually, the prompt line and many learner notes make it sound identical to disillusioned. The difference is grammar and focus: disillusioned describes the person; disillusionment is the loss itself. You do not say “I am disillusionment.”