Disillusion is defined as disappointment that you feel when you realize something you thought was true wasn't, or when you realize that something you thought was good is not as good as you believed it was.
An example of disillusion is what you feel when you find out that Santa isn't real.
(countable) The act or process of disenchanting or freeing from a false belief.
But however indubitable that conclusion and the officer's conviction based upon it, Pierre felt it necessary to disillusion him.
In order to disillusion anyone who may think that my position was a sinecure, I shall now digress.
The mutual ardour;gradually cooled; motives of prudence and decorum urged the discontinuance of the connexion; and disillusion changed insensibly to disgust.
The result was that, despite widespread popular disillusion with New Labor this June, we failed to move public opinion our way.
No event in the history of England had been attended with more lively and general rejoicing than Charles's restoration, and none was destined to cause greater subsequent disappointment and disillusion.