a variable is a symbol used to represent for any one of a set of numbers. the set is called the range of the variable. variables are values that change or vary. (hence the name variable). variables may be reperesented by letters such as "x", "y", "a", "b" etc.
constants are values that do not change. they are represented by symbols like "3", "10" and so forth.
these terms stand in contrast to each other and are used in mathematics and computer science.
