Question

Define a variable. How does a variable relate to a constant?

Answer 1

In elementary mathematics, a variable is an alphabetic character representing a number, called the value of the variable, which is either arbitrary or not fully specified or unknown

Answer 2

By definition, Variable is "(of a quantity) able to assume different numerical values." By this, an example is 4x, as x is an unknown number, but has a value.
Constants are one constant value, unlike Variables.