Dy is the differential of function y, and note that δ y is the increment of function.

Ordinary, dy? ⊙δy .

The definition of differential in mathematics: from the function B=f(A), two groups of numbers A and B are obtained. In A, when dx approaches itself, the limit of the function at dx is called the differential of the function at dx, and the central idea of the differential is infinite division.

Differential is the linear main part of function change. One of the basic concepts of calculus.