Fallacies of definition refer to the various ways in which definitions can fail to have merit. The term is used to suggest analogy with the logical fallacies. This is a typical sort of list found in texts used in college logic courses.
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If one concept is defined by another, and the other is defined by the first, we have a pair of circular definitions, somewhat similar to a question-begging argument: neither offers us any enlightenment about what we wanted to know.
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—by just using the word "excellence". It might be perfectly true that all virtues are excellences and all excellences are virtues, but the word "excellence" 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 may also have a basic philosophical confusion about what "excellence" might mean.
A definition does 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 he or 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.
A definition is too broad if it applies to things that are not part of the extension of the word defined. Suppose one defines 'bachelor' as 'unmarried male'. At first glance this might look correct, but male is a word that can apply to many things. For example, unmarried male dogs and unmarried male babies are not considered bachelors. Narrowing the definition can avoid this problem. In this case, 'bachelor' can mean 'unmarried man'. However, the Pope, or a man whose wife has died and who has not remarried, are not considered bachelors either — so if it is to fit the common conception, the definition must be further narrowed to 'a man who is socially regarded as able to marry, but has not yet'.
Definitions are considered "over-narrow" when they exclude certain aspects, characteristics or applications of the term, thereby falling short of a full and true definition as they fail to describe particular 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 in them (like a chest of drawers) 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. 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 seemingly disparate characteristics strung together by 'or' are probably not truly describing a single concept.
Definitions can go wrong by using ambiguous, obscure, or figurative language. Suppose we defined 'love' as 'the insensible quivering of the soul'. Given a definition like this, one might ask: what is the insensible quivering of the soul? How would we recognize it? Is Johnny's soul insensibly quivering right now? To be useful, worthy of further discussion, definitions must be stated in plain, straightforward language that can be understood by the people to whom the definitions are given. Even an atrocious definition will elicit a better definition. See jargon.
An often quoted example is Samuel Johnson's definition for oats: "Oats: a grain which in England is generally given to horses, but in Scotland, supports the people", to which his Scots friend, Lord Elibank, retorted, "Yes, and where else will you see such horses and such men?"
Examples are by themselves insufficient to define a term. For instance: "Chutzpah is killing your parents and then throwing yourself on the mercy of the court citing that you are an orphan." Such a definition is only useful if the reader has some larger context for the term being defined.
Even a series of examples may be inadequate. For example, "Edentates are mammals such as voles and tenrecs," does not help the reader to decide if, say, shrews are edentates or not, even if the reader knows what voles and tenrecs are.
Finally, combination of definitional fallacies can create complex definitions that are fallacious; example: "Situationism is the intellectual framework of situationist movements such as the student protests in France in 1968." If we remove the example, the remaining definition is circular. So this definition does not serve the needs that definition should, such as being able to determine if say, Ann Coulter were a situationist.