The underlying rhetorical structure of the definition argument is x is/is not y. Review the following examples of definition claims and notice that they are all concerned with saying what something is (or isn't) and assigning it to a category (or excluding it).
Sometimes a claim can look like a definition claim, but it actually expresses a fact rather than a stand on one side of an argument. All the following examples are statements of fact and are NOT definition claims. It's simply impossible (under ordinary circumstances) to argue about these facts.
Occassionally, students slip out of their definition arguments and into some other kind of argument. The following examples are NOT definition claims.
As you attempt to formulate your own definition
claim, ask yourself the following two questions: 1) Is my thesis a definition
claim or have I lapsed into some other rhetorical form? 2) Is my thesis
arguable? If my thesis is arguable, I should be able to think of
reasons why my opponents disagree with it.