Surely, if one has a basic confusion about what 'virtue' means, then one will also have a basic philosophical confusion about what 'excellence' means. So it will not do to define one simply by stating the other. A definition is no good if it uses a very near synonym in the definition. For example, suppose we define 'beautiful' as 'possessing aesthetic value'. The question is what general characteristics are possessed by all beautiful objects, or all objects that have aesthetic value.
Definitions can be too broad .
Suppose we define 'bachelor' as 'unmarried male'. On first glance this might look all right, but it applies to a lot of things, for example, male dogs, and male babies, that, needless to say, are not bachelors. A definition is too broad if it applies to things that are not part of the extension of the word defined. Definitions can be too narrow . So even though some pieces of furniture are objects that are used to sit on, not all furniture is used to sit on.