Definition:Free Variable/Predicate Logic

Definition
In predicate logic, a free variable is a variable which exists in a WFF only as free occurrences.

Also known as
A free variable is often referred to as an unknown, particularly in mathematical contexts.

In the field of logic, a free variable can also be referred to as a real variable.

However, this can be confused with a variable whose domain is the set of real numbers, so its use on is discouraged.

The name arises in apposition to the name apparent variable, which is another name for bound variable.

Also see

 * Definition:Free Occurrence: a somewhat more precise concept, recognising the fact that a variable may appear multiple times in an expression, and not necessarily always of the same category.