Chapter 4 - Applications of the Derivative - 4.1 Linear Approximation and Applications - Preliminary Questions - Page 172: 4

The change in the input $\Delta x $ (when $\Delta x $ is small).

The Linear Approximation shows that up to a small error, the change in output $\Delta f $ is directly proportional to the change in the input $\Delta x $ (when $\Delta x $ is small).