Fallacies of definition
Ways in which a term may be poorly defined
Fallacies of definition are the various ways in which definitions can fail to explain terms. The phrase is used to suggest an analogy with an informal fallacy. Definitions may fail to have merit, because they are overly broad, overly narrow, or incomprehensible; or they use obscure or ambiguous language, contain mutually exclusive parts, or (perhaps most commonly) are circular.
Circularity
If one concept is defined by another, and the other is defined by the first, this is known as a circular definition, akin to circular reasoning: neither offers enlightenment about what one wanted to know. "It is a fallacy because by using a synonym in the definiens the reader is told nothing significantly new."
Incongruity: overly broad or narrow
A definition intended to describe a given set of individuals fails if its description of matching individuals is incongruous: too broad (excessively loose with parameters) or too narrow (excessively strict with parameters). For example, "a shape with four sides of equal length" is not a sufficient definition for square, because squares are not the only shapes that can have four sides of equal length; rhombi do as well. Likewise, defining rectangle as "a shape with four perpendicular sides of equal length" is inappropriate because it is too narrow, as it describes only squares while excluding all other kinds of rectangles, thus being a plainly incorrect definition.
If a cow were defined as an animal with horns, this would be overly broad (including goats, for example), while if a cow were defined as a black-and-white quadruped, this would be both overly narrow (excluding: all-black, all-white, all-brown and white-brown cows, for example) and overly broad (including Dalmatians, for example).
Obscurity
Definitions can go wrong by using ambiguous, obscure, or figurative language. This can lead to circular definitions.
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