In the context of unification, a metavariable is a free variable that stands for an unknown term. Metavariables are also known as "unification variables" and "type variables", although the latter is often ambiguous with polymorphism.
In many materials there is no syntactic distinction between metavariables and bound variables. This means that given a variable, the reader must determine which of the two it is. However, in programming the distinction is often made more obvious. For instance, Prolog requires metavariables to start with a capital letter.