"Equation" is also called "equation" or "system of equations", that is, an equation with unknown numbers. For example: x-2=5, x+8=y-3. The value of the unknown quantity in the equation is called the "solution" or "root" of the equation. The process of finding the solution of the equation is called "solving the equation".

There are many kinds of equations. In algebra, according to the number of unknown equations, it can be divided into one-dimensional equations, two-dimensional equations, three-dimensional equations and so on. According to the highest number of unknown terms in the equation, it can be divided into: linear equation, quadratic equation, cubic equation and so on. There are differential equations, difference equations and integral equations in modern mathematics. In addition, the equation can be divided into linear equation and nonlinear equation.

In natural science, a special formula is usually used to express the process of mutual transformation between microscopic particles under specific conditions. This formula is also called "equation" or "equation" for short. For example, nuclear reaction equation, chemical equation, thermochemical equation, biochemical reaction equation, equation about the generation and annihilation of microscopic particles, etc.