Fallacies of definition
||It has been suggested that this article be merged into Informal fallacy. (Discuss) Proposed since April 2012.|
If one concept is defined by another, and the other is defined by the first, this is known as circular definition, somewhat similar to a circular reasoning: neither offers us any enlightenment about what we wanted to know.
Overly broad or narrow definitions
A definition is no good if it defines its subject with overly wide or narrow parameters. For example, "a shape with four sides of equal length" is not a good definition for 'square'. Why? Not only squares have four sides of equal length; rhombi do as well. This is proven by identifying not only the term being defined, but also the conditions in the definition. Likewise, defining a 'rectangle' as "a shape with four perpendicular sides of equal length" is not valid because it is too narrow. 
Definitions can go wrong by using ambiguous, obscure, or figurative language. If 'beauty' is defined as 'aesthetically successful', one must continue to break down and define the following definition. This can often lead to circular definitions. Definitions should be defined in the most prosaic form of language to be understood. Failure to elucidate provides fallacious definitions.
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?"
Self-contradictory statements in context
Often statements can contradict themselves due to a difference in definitions while defining something. For example, the statement "A society is free if and only if liberty is maximized and people are required to take responsibility for their actions" is true or paradoxical, depending on the individual's definition of liberty. If liberty is taken to mean "the ability to exercise one's rights as provided for by the law and nature" then this is true, but if it means "the state when one is not held to nor required to perform anything against their will" then this is clearly false.