In mathematics, D has many meanings. For example, D can represent an unknown number or the diameter of a circle, R is the radius of a circle, and it also has the meaning of the coefficient of a linear term in a quadratic function. In addition, it also represents a constant term in a linear function. In mathematical derivatives, d is the operator, D=d/dx, Df=df/dx, that is, the derivative.

In the derivation, the source of D is originally difference = gap. When the gap approaches zero infinitely, it becomes differential, which means infinitesimal, and it is called differential. "Differentiation" is a process, an endless process of "dividing" and "dividing".