In semantics, the metalanguage is the language that is used to talk about (expressions of) another language, the object language.
Object language and metalanguage can be the same, e. g. ordinary English, but they always differ in function. If we cannot detect any metalanguage in a sentence, there is no object language either. The distinction was introduced to avoid the liar's paradox.
Gamut, L.T.F. (1991) Logic, language, and meaning, Univ. of Chicago Press, Chicago.