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.
A circular definition is a fallacy because if the meaning of a term is unknown, one cannot use in the definition the same unknown term. Using another unknown term should also be forbidden. Nevertheless, that is how dictionaries ultimately work, which proves the existence of undefined terms, terms whose meanings we know but cannot define further. This result lead to what is known in mathematics as intuitionism.
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.
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.
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.