Explanation: The definition of a term includes items which should not be included.
- 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.)
- 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.