Which biconditional is NOT a good definition?

A biconditional that is not a good definition is one that is too broad, ambiguous, or relies on terms that themselves need defining. For example, stating "A shape is a circle if and only if it is round" is not a good definition because "round" is not a precise term and can lead to confusion about the specific properties that define a circle. A good definition should be clear, concise, and use terms that are well-defined within the context.