A logical fallacy in which the definition of a term includes the term as part of the definition. Note that a clarification is not necessarily a circular definition; for example, this definition for 'circular definition' includes the word 'definition', but it is not circular because it is clarifying a particular type of definition. However, some definitions may be mutually circular: follow the definition in Fastolfe's writeup above.

Example: "A website is pornographic if it contains pornography."

To prove the definition circular, identify the conditions of definition and show that at least one term in the conditions is the term, or is defined in terms of the term (or is defined in terms of something defined in terms of the term, etc.).