Standard distinction, applied to linguistics by Danish linguist Louis Hjelmslev (1899-1965).
Also discussed by Roman Jakobson (1896-1982).
Language about language: metalanguage is a system of notation, descriptive terms, and so on, for an 'object language'. Metalanguage may be related to natural language - terms like 'passive', 'auxiliary' -or an abstract notation as in symbolic logic.
Also see: GLOSSEMATICS, POETIC PRINCIPLE
J Lyons, Semantics (Cambridge, 1977)