The definition includes the term being defined as a part of the definition. Circularity is a special case of a Failure to Elucidate.
At some point, @MerriamWebster added a perfectly circular, tertiary definition to accommodate the new regime. “Female is the opposite gender identity as male.” Surely you’re wondering, as I did, how bald-faced is the circularity? Will “male” simply mirror this language game, equally devoid of reference to reality? Yes. “Male is the opposite gender identity as female.”Nathan Jacobson, Twitter.com (October 20, 2021).
On the definition of equality
So there it is: equality is entirely “[c]ircular.” It tells us to treat like people alike; but when we ask who “like people” are, we are told they are “people who should be treated alike.” Equality is an empty vessel with no substantive moral content of its own. Without moral standards, equality remains meaningless, a formula that can have nothing to say about how we should act. With such standards, equality becomes superfluous, a formula that can do nothing but repeat what we already know.Peter Westen, “The Empty Idea of Equality” in The Harvard Law Review , Vol. 95, No. 3 (January, 1982).
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.