To disillusion someone is to make them see that a thing they believed was true or good is not. You take the pretty picture away. A child who thought every hero was kind may be disillusioned by a real story; a voter may be disillusioned by a leader who does not keep promises. It is stronger and colder than disappoint. Disappoint is “I hoped for a gift and did not get it.” Disillusion is “I thought this person, this job, this country was a certain way — and I found out I was wrong.” The verb is the usual learner use: The film disillusioned her. The same spelling is also a noun, a little more formal, for that loss of belief itself: a growing disillusion with politics. Teachers still prefer disillusionment for the noun, but you will meet both. You disillusion a person; you do not usually disillusion a plan.