I would assume that "functional definition" means the way a word or term is used and understood in common language and use. This is not always the same as a dictionary's definition. For instance, one dictionary definition of "brainwashing" is "The application of a concentrated means of persuasion, such as an advertising campaign or repeated suggestion, in order to develop a specific belief or motivation. "
However, that is not what a person usually thinks of when someone uses the term brainwashing. So the functional defintion would be different than the dictionary's definition in this case.
Am I wrong?