Algebraic expressions
and formulae, such as y = x^{2}, a^{2} + b^{2} = c^{2}
and v = u + at, contain
letters which can stand for many different numbers. These letters
are the variables. Variables are useful in mathematics because they
allow instructions to be specified in a general way. If one were
forced to use actual values, then the instructions would only apply
in a more narrow set of situations.

For example:

Specify a mathematical definition for finding the number twice that
of any number : double(y) = y + y