If there is only one variable in the equation, then it is called a linear equation in one variable. ax + b = c, where a, b and c are real numbers. These are called linear equations in one variable because the highest degree of the variable is one and there is only one variable. Every linear equation in one variable has a one and unique solution. If the equation has two or more variables then it becomes a linear equation in two variables or linear equations in three variables and so on and the number of solutions varies as per the count of variables an equation contains.