What is a dimension in math?

In a mathematical equation the dimension is the number of variables that are allowed to vary or that determine the result of the equation.

In the equation Y = 4X + 35 ==> the variable Y is the dependant variable and the variable X is the independent or free component (allowed to vary) and the equation has one dimension or degrees of freedom.

In the equation Y = 13A + BC + 15AD + XZ/13 + 1092 ==> The variable Y is the dependant variable and the 6 variables A, B, C, D, X and Z are the independent or free components (allowed to vary) and the equation has 6 dimensions or degrees of freedom.