It is usually regarded as its most distinctive feature.
Language as a Structure
Development in Lacan's Thought
Language is first and foremost an appeal to an interlocutor.
Language and Structure
Lacan and Language
Properly speaking this is a redundancy because "structured" and "as a language" for me mean exactly the same thing. Structured means my speech, my lexicon, etc., which is exactly the same as a language. And that is not all. Which language? Rather than myself it was my pupils that took a great deal of trouble to give that question a different meaning, and to search for the formula of a reduced language. What are the minimum conditions, they ask themselves, necessary to constitute a language? Perhaps only four signantes, four signifying elements are enough. It is a curious exercise which is based on a complete error, as I hope to show you on the board in a moment. There were also some philosophers, not many really but some, of those present at my seminar in Paris who have found since then that it was not a question of an "under" language or of "another" language, not myth for instance or phonemes, but language. It is extraordinary the pains that all took to change the place of the question. Myths, for instance, do not take place in our consideration precisely because those are also structured as a language, and when I say "as a language" it is not as some special sort of language, for example, mathematical language, semiotical language, or cinematographical language. Language is language and there is only one sort of language: concrete language — English or French for instance — that people talk. The first thing to start in this context is that there is no meta-language. For it is necessary that all so called meta-languages be presented to you with language. You cannot teach a course in mathematics using only [[[letter]]s on the board. It is always necessary to speak an ordinary language that is understood. 
- Lacan, Jacques. Écrits. Paris: Seuil, 1966. p.82
- Lacan, Jacques. The Seminar. Book XI. The Four Fundamental Concepts of Psychoanalysis, 1964. Trans. Alan Sheridan. London: Hogarth Press and Institute of Psycho-Analysis, 1977. p.20
- Lacan, Jacques. Le Séminaire. Livre XX. Encore, 1972-73. Ed. Jacques-Alain Miller. Paris: Seuil, 1975. p.126
- Of Structure as an Inmixing of an Otherness Prerequisite to Any Subject Whatever
- Language: 12, 24-5, 33, 44-5, 71, 83, 118, 119, as system, 38, 40 (35, 37)
- Lacan, Jacques. The Seminar of Jacques Lacan, Book XX: Encore, On Feminine Sexuality, The Limits of Love and Knowledge 1972-1973. Trans. Bruce Fink. New York: W. W. Norton & Company, 1998.