the symbolic order in Lacan's subject theory is also referred to by him as the Big other or simply the Other with a capital O (to distinguish from the other of the imaginary order). Lacan's ideas regarding the symbolic order or the big other are based on the works of de-Saussure, Jakobson, Hegel, Heidegger and Levi-Strauss. Lacan, like Levi-Strauss, held that the social world is constructed through rules designated to regulate various forms of personal relations and exchange. The most basic form of interaction and exchange for Lacan is verbal communication which is the base for Lacan's symbolic order.
Like Freud before him, Lacan attributed great importance to speech during psychoanalytic therapy. Lacan held that the subject is the product of language (Foucault would "upgrade" this definition to the subject being a product of discourse), and therefore argued that spoken language is not only the main instrument of therapy but also the agent which establishes the individual's reality and reality in general. Everything which is beyond language, that cannot be spoken through language, is termed by Lacan as the real. Language for Lacan points to an inherent absence in it, for it is something that always comes to replace something real. In language we exchange objects for words, thus eliminating objects. It is this process of symbolization which allows for human communication and understanding. The abstraction of language is what enables mutual accord regarding meaning. Therefore language is not just about relaying information, it also has the function of appealing to the other. But since language precedes the individual he experiences it with a sense of alienation. Therefore language for Lacan is the symbolic order or the "big other". The big other or symbolic order for Lacan is universal in that that it originates from outside the person. This "otherness" of language is what allowed Lacan to establish the indevidual's alienation within it.
Suggested reading on and by Lacan: