History of letter $h$ in the formulation of differentiability

Question

$f$ is differentiable in $x$ if $\lim_{h \rightarrow 0} \frac{f(x+h) - f(x)}{h}$ exists. Why is the letter $h$ typically used here, what it the history of this? I feel that $\delta$ or $\Delta$ for example would better fit.