To cause someone to realize that something isn't as good as they believed; to free from illusion.
Third-person singular present or plural of disillusion, from dis- (remove) + illusion (false belief). The verb form became standard in English by the 19th century.
As a verb, 'disillusions' is simpler and more direct than 'disillusionment' the noun—'He disillusions her' is more active and personal than discussing disillusionment in abstract terms, which is why it appears often in literature about relationships and education.
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