John bought a house. He decided to reduce his square backyard by 40 inches along its width. The area of the new backyard is given by the function below, where z represents the length in inches.
A(z)=z^2-40z
Which statement best describes the term 40z?
* The area of the playground after expansion
* The width of the backyard
* The area of the backyard before reduction
* The area reduced from the backyard