The definition is harder to understand than the term being defined.

Examples

1. Someone is lascivious if and only if he is wanton. (The term being defined is "lascivious". But the meaning of the term "wanton" is just as obscure as the term "lascivious". So this definition fails to elucidate.)
2. An object is beautiful if and only if it is aesthetically successful. (The term "aesthetically successful" is harder to understand than the term "beautiful".

Proof

Identify the term being defined. Identify the conditions in the definition. Show that the conditions are no more clearly defined than the term being defined.