In order to write a good definition, you need to do three things. First, give the word to be defined. Second, give the general class to which your subject belongs. Third, give the particular characteristics of your subject. In other words, show how your subject is different from the rest of the members of its class.
Suppose that you want to define an owl. You would begin your definition by stating that an owl is a bird. You have given the words and its general class. A bird, however, might be a pigeon, penguin, or swan. To define this bird you might add it is a bird of prey, which means a bird that kills other animals for food. By using this specific information you might write a good definition. You should include a good definition as part of the introduction of an explanatory what composition. Your definition serves as the topic sentence for the entire composition. Be sure that your definition is clear and specific.