Any subject can be said to have a meta-theory, a theoretical consideration of its properties, such as its foundations, methods, form and utility, on a higher level of abstraction. In linguistics, a grammar is considered as being expressed in a metalanguage, language that operates on a higher level in order to describe properties of the plain language (and not itself).
Meta is also gaining currency as an adjective, as well as a prefix, as in the work of Douglas Hofstadter.