In mathematics, a variable is a value which usually is the subject or part of a function and must be subject to change while not affecting the structure behind the function in which it acts.
For example, if we have a function: 
, then 
are said to be the variables of the function, 
.
Further, 
. We change the variable and thus alter the value of the function.
The set of all variables upon which a function acts is said to be the domain. The set of all values that a function can take due to its domain is called its range.