1. a language that can be used to describe languages;
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 (r) 3.0 (2006):
n 1: a language that can be used to describe languages
The Free On-line Dictionary of Computing (30 December 2018):
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.