Definition:Circular Definition

Definition
A circular definition is a definition of a concept $A$ in terms of another concept $B$ such that the definition of $B$ itself refers (either directly or indirectly) to the definition of $A$.

It is a truism that circular definitions are always to be avoided.

Example

 * "An integer is an element of an isomorphic copy of the inverse completion of the set of natural numbers."
 * "A natural number is a positive integer."

Also see

 * Circular Proof
 * Undefined Term