Definition demonstrates how to set the terms or parameters of an argument. Defining issues in terms that support your position frames the argument so that, through syllogistic reasoning, an audience can be lead logically to the conclusion you intend. To argue by definition, then, is to convince the audience that the definitions are reasonable, supportable and logical and, since your argument is based on them, your conclusions are as well.
In an editorial arguing for dismissing a given professor, the writer begins by defining what makes a "good" teacher: knowledge of topic, interest in student learning, a teaching style that holds students' attention, an ability to explain clearly difficult concepts, availability for conferences with students, and fair evaluation methods. Once a good teacher is defined in this way, the author can then demonstrate how Professor X has none of these qualities, proving his judgment with evidence at each point from student evaluations, interviews, etc. Logically, then, if Professor X does not fit the definition of a good teacher, the readers will reach the conclusion that he is a bad one and should be dismissed.