One idea for a definitional argument is to simply define a term. For this to be considered a definitional argument rather than a categorical argument, however, the subject must be controversial. Choose a topic that many people in your intended audience will likely disagree with. For example, "Define childhood obesity as child abuse." Many people do not agree with this claim. By asserting in your argument that allowing your child to become obese is a form of abuse, and then backing up that claim with evidence about the lifelong consequences of obesity, the effects on the child's future health and failing to maintain nutritional standards as parental neglect, you are making a definitional argument.
Another form of definitional argument is to support an accepted definition of a term. Again, for this to be considered a definitional argument, the topic must be somewhat controversial. An example of this type of argument would be choosing a famous sports player who has been ranked as the best in his sport. This is controversial because although one organization or individual may have publicly ranked this player as number one, other organizations and individuals may disagree. Back up the top ranking with the player's statistics. Compare your chosen player with other players to show how and why he is the best in his sport.
A third way to form definitional arguments is by contradicting an agreed-upon definition. Choose a somewhat controversial topic for this argument to be effective. One example of this would be to argue that sounds and silences in nature fall under the category of music. Though the definition of music has long been argued and contested by musicians and non-musicians, most musicians agree that music consists of sounds and silences organized through time, usually by a human composer. Make an argument for why natural sounds and silences can also be considered music according to this definition.
Regardless of which type of definitional argument you choose, the format you make it in is generally the same. Most definitional arguments begin with a discussion of the controversial nature of the topic. Cite controversies about your topic to introduce it to the reader or listener. State your claim next, then back it up with at least three pieces of evidence. Following that, summarize and refute two to three possible arguments against your claim. Then conclude your argument.