"Homer Defined" has received generally positive reviews from critics. The authors of the book "I Can't Believe It's a Bigger and Better Updated Unofficial Simpsons Guide", Warren Martyn and Adrian Wood, described it as an excellent episode which added new depth to the show in the scene with Marge trying to convince Luann to let Milhouse play with Bart again. They added that Lisa's "faith in her heroic father makes a nice change", and said that the episode's ending, in which Homer enters the dictionary, "is most satisfying".