The Collaborative International Dictionary of English v.0.48:

metalanguage \metalanguage\ n.
   Any language that can be used to describe another language or
   system of symbols.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

metalanguage
    n 1: a language that can be used to describe languages

The Free On-line Dictionary of Computing (30 December 2018):

metalanguage

   1. [theorem proving] A language in which proofs are
   manipulated and tactics are programmed, as opposed to the
   logic itself (the "object language").  The first ML was
   the metalanguage for the Edinburgh LCF proof assistant.

   2. [logic] A language in which to discuss the truth of
   statements in another language.