Meta-variable

In semantics, a meta-variable is an expression of a metalanguage that can be used to refer to arbitrary object language expressions in a logical language.

EXAMPLE
The syntax definition of a predicate logic could contain sentence (i).

(i) If phi is a formula in L, then so is Neg phi

In (i), phi is a metavariable standing for any formula of the logical language L.

Link
Utrecht Lexicon of Linguistics

Reference
Gamut, L.T.F. (1991) Logic, language, and meaning, Univ. of Chicago Press, Chicago.