Starting an essay with a definition isn't imaginative because it's been done too many times over and isn't engaging because,well when was the last time anyone has cracked open the dictionary and said,"Hey I found this really interesting definition today!" never. It's just not a very professional way to start an essay and many English teachers will agree.
The word "dream" emphasises the longing which the persona holds that is not quite tangible - the repetition implies this "dream" is reoccurring, showing that the persona's longing is all the more strong.