 One way to specify the nature of a category is to note the theoretical role it plays or the role played by items within it. This is to say what items within the category do. Another is to give a real definition of the category (or the items within it), where a real definition attempts to identify the real essence of the members of the category. This is to say what items of within category are (see entries on  theoretical terms in science  and  Locke on real essence  and the  section on real and nominal definitions in the entry on definitions).