D means infinitesimal, which is called differential in mathematics. dy is infinitesimal for Y, dx is infinitesimal for X, and dy/dx is the ratio of two infinitesimals, that is, the rate of change of Y to X, also called derivative function for X, or derivative for short.

Derivative: It is an important basic concept in calculus. When the increment of the independent variable tends to zero, the limit of the quotient of the increment of the dependent variable and the increment of the independent variable, and a function has a derivative, it is said that the function is derivable or derivable and the derivable function must be continuous. Discontinuous functions must be non-differentiable, and derivatives are essentially a process of finding limits. The four algorithms of derivative are derived from the four algorithms of limit.