For example, suppose that the variable a represents the number 12. Then the value of a is 12. For other variables, you may start out not knowing the value of the variable, but in the course of solving a problem, you determine the value. In any case, it is important to keep track of what values a variable may have. For example, a variable which represents an area cannot be negative.
Many problems involve units, such as feet, centimeters, years, pounds, and liters.
It is important in the definition of a variable to state the units in which the variable is expressed. For example, suppose that A represents the area of a circle, where the radius is given in meters. For these, we just omit the units from the definition of the variable.