Is it okay to start a formal essay with a dictionary definition?
If I were in your position, I would use the dictionary definition when in the body paragraph. I think that it is a good idea to use a dictionary definition [Thanks for this idea! XD], but it would be more suitable in the body due to the fact that you only introduce about the essay you’re writing, therefore a dictionary definition is irrevalant in the introduction. You could use the dictionary definition as a support to a topic sentence.