When there is a functional relationship between two variables, there is a linear relationship between them. Positive proportional relationship is a special case of linear relationship, while inverse proportional relationship is not linear relationship. More generally speaking, if these two variables are taken as the abscissa and ordinate of a point respectively, and they look like a straight line on the plane, then the relationship between these two variables is linear. That is, if a binary linear equation can be used to express the relationship between two variables, the relationship between these two variables is called linear relationship, so the binary linear equation is also called linear equation. By extension, the linear equation with n variables is also called linear equation with n variables, but it has nothing to do with straight lines.