Definitions can go wrong in various ways; for convenience one can refer to these as
fallacies of definition (by analogy with the
logical fallacies). This is a typical sort of list found in texts used in
college logic courses.
There is a general name for the first two sorts of error: circular definition. A circular definition is somewhat similar to a question-begging argument: neither offers us enlightenment about the thing we wanted to be enlightened about.
A definition is no good if it simply gives a one-word synonym. For example, suppose we define the word 'virtue'--an important word in
ethics--just using the word 'excellence'. It might be perfectly true that all virtues are excellences and all excellences are virtues (that was perhaps an ancient Greek view; see
arete), but the word 'excellence' by itself is
not a good definition of "virtue," in philosophy. One can always simply ask, "But what does 'excellence' mean?" 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 words 'beautiful' and 'aesthetic' are very nearly the same in meaning; so if anyone is deeply confused or curious about beauty, then she is of course going to be confused or curious about the aesthetic. 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.
To correct this fallacy, narrow the definition.
In this case, 'bachelor' can mean 'unmarried adult male living sentient being'.
Of course, some of us feel that this alternate definition of 'bachelor' is viciously pleonastic.
Definitions can be
too narrow. That is, they can
exclude some things that they
should apply to; they fail to describe some members of the word's
extension. Here is an example of a narrow definition: 'piece of furniture' means 'object used to sit on'. Of course, some pieces of furniture are not used to sit on; for example, we put objects on them (like tables) or we put our feet on them (like footstools), and so forth. So even though
some pieces of furniture are objects that are used to sit on, not
all furniture is used to sit on. We need a broader definition: we might add other qualifying characteristics, like 'used to put feet up on' or 'used to put household objects on', for example. That would make the extension of the definition bigger--that is, the definition would apply to more things, and more of the things that we use the word 'furniture' to describe. We might also choose to entirely rewrite the definition, since laundry lists of characteristics strung together by 'or' are generally regarded by philosophers as not describing a unitary concept.
Definitions can go wrong by using ambiguous, obscure, or figurative language. Suppose we defined 'love' as 'the insensible quivering of the soul'. This is useless. Given a definition like this, one has the right to ask: but what is the insensible quivering of the soul? How would we recognize it? Is Johnny's soul insensibly quivering right now? And so on. Definitions should be stated in plain, straightforward language that can be understood by the people to whom one is giving the definition. See jargon.