Summary

The definition includes the term being defined as a part of the definition. (A circular definition is a special case of a Failure to Elucidate.)

Examples

1. An animal is human if and only if it has human parents. (The term being defined is "human". But in order to find a human, we would need to find human parents. To find human parents we would already need to know what a human is.)
2. A book is pornographic if and only if it contains pornography. (We would need to know what pornography is in order to tell whether a book is pornographic.)

Proof

Identify the term being defined. Identify the conditions in the definition. Show that at least one term used in the conditions is the same as the term being defined.