Definition:Differentiation

Definition
The process of obtaining the derivative of a differentiable function $f$ with respect to $x$ is known as:
 * differentiation (of $f\,$) with respect to $x$

or
 * differentiation WRT $x$

Also known as
The technique of differentiation was called the method of fluxions by.

The word fluxion evokes the concept of quantities flowing towards zero. Hence the implicit dependence upon continuity.