The variation of a function means the rate at which a certain function changes. The rate of variation of a function is also called the slope.

**According to the mathematical definition, the slope represents the change of the function** **$(Y)$**** by increasing the value of** **$X$**** by** **$1$****.**

- If the function's graph is represented by a straight line, it means that the rate of variation of the function is constant
- However, if the graph is not represented by a straight line, this implies that the rate of variation of the function is not constant