Definition:Variable

Definition
A variable is a label which is used to refer to an unspecified object.

A variable can be identified by means of a symbol, for example, $x, y, z, A, B ,C, \phi, \psi, \aleph$. It is often convenient to append a subscript letter or number to distinguish between different objects of a similar type:


 * $a_0, a_1, a_2, \ldots, a_n; S_\phi, S_{\phi_x}, \ldots$

The type of symbol used to define a variable is purely conventional. Particular types of object, as they are introduced, frequently have a particular range of symbols specified to define them, but there are no strict rules on the subject.

Domain
The collection of all possible objects that a variable may refer to has to be specified.

This collection is the domain of the variable.