Definition:
The derivative of a function $f$ at a number $a$, denoted by $f'(a)$, is the value
$$f'(a)=\lim_{x \to a} \frac{f(x)-f(a)}{x-a},$$
if this limit exists.
Besides being a rate of change, $f'(a)$ is the
slope of the tangent line to $y=f(x)$ at $(a, f(a))$, and is the
conversion factor from small changes in $x$ to small changes in $f(x)$.