The definition includes items which should not be included.

Examples

1. An apple is something which is red and round. (The planet Mars is red and round. So it is included in the definition. But obviously it is not an apple.)
2. A figure is square if and only if it has four sides of equal length. (Not only squares have four sides of equal length; trapezoids do as well.

Proof

Identify the term being defined. Identify the conditions in the definition. Find an item which meets the condition but is obviously not an instance of the term.