A doctorate given to someone who didn't do any of the traditional coursework or research that most people need to do. Most honorary doctorates seem to go to celebrities and politicians who, however great their real-life accomplishments may be, aren't even remotely qualified to be hooded.
In one of his books, Richard Feynman talks about the resentment he felt when, before he got his Ph. D., he had to wait for the honorary doctorates to be passed out like party favors. After all, he had to work long and hard for many years. The story goes that he promised himself then that, should the opportunity arise, he would never dilute true academic accomplishment by accepting an honorary doctorate and, later on when he became famous and renown, he kept his word.