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1.
Stop. There is no such thing of having Illusions of Grandeur, because an Illusion means that you're seeing something that is actually real, but is such that is easily misinterpreted (like any numerous optical illusions- Think of looking at a mirror that makes you look super tall at a circus or fair)

Delusions of Grandeur however is when a person believes something that isn't true, and is usually grand... like them being the savior of the world, a secret agent, or a successful business man or woman, etc.
Suzy goes on and on at school about how she's secretly dating British Royalty on the side.. and how the Prince wants to marry her, but she's not sure if she could handle all the attention and she balks at having to put on airs, do charity work, and all the other pretentious crap that would go along with being the wife of a prince. Suzy would be having Delusions of Grandeur NOT Illusions of Grandeur.
by ann weiss November 03, 2010
 
2.
An Illusion is the state of being deceived. Grandeur is the quality of being impressive or awesome. Therefore, someone who has Illusions of grandeur is deceiving themselves into thinking they are impressive or awesome in some way. In reality, no one else sees them in this way.
Ann Weiss thought she knew what she was talking about - like she was some authority on English or something - but in reality, she was having illusions of grandeur.
by ProphetJoe November 30, 2011