Definition:Variable/Predicate Logic

A variable is a label which is used to refer to an unspecified object. The collection of all possible objects that the variable may refer to will need to be specified.

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.