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,
Circularity
thumb|Circular definition of inflammable liquid
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." The thing defined (oats) should be pointed out rather than remain obscure.
Mutual exclusivity
The definition completely excludes what is being defined. For example, a cow might be defined as a flying animal with no legs.