The Symbolic, Imaginary and Real
Some beginning definitions
Imaginary, symbolic, real: three "registers", aspects or quasi-functions by which speech and thereby perception, actions and the body are influenced or dominated by different structures of language. As an analogy think of the way in which the respiratory, circulatory and neurological systems interact with one another to influence or determine the state of the body. A crucial difference, as we see in the Seminar on "the Purloined Letter", is that these structures must be understood intersubjectively, that is, between persons, not as mere structures of individual persons.
The imaginary register of speech, the signifying chain broadly understood so as to include objects as signifiers, is an eroticizing of the relation between subjects or, more broadly, between the subject and the object. Erotic sex is a metaphor, not the essence of, for the logic of this eroticization. (Sex itself may be either deeroticized or eroticized.) The imaginary constitutes the ways in which relations which at the level of consciousness appear to be relations between detached things are, at the level of the poetics of the unconscious, projected as relations of belonging to, being part of, even fused with, one another. But crucially, for Lacan, this is an effect of "speech acts," involving a distinctive rhetoric of metonymy and feint. (This is a point at which to appreciate the important of Kristeva's work, for in her sense, the semiotic has to do with the way in which desire may be carried in more "primitive," less structural dimensions of speech, albeit not without being related to more symbolically defined aspects. Thus, one may perceive desire in what might be called certain quasi-musical features of speech, pace, tone, rhythm, phonematic anomalies of various sorts, etc.)
The symbolic register of speech is its bearing witness to and submitting itself to an order of law, of pact and of contract. This includes the rules and conventions of language as well as the use of words to formulate laws, legal relations more specifically, and tacitly accepted taboos, etc. . The symbolic order is what a photograph of these laws at a particular point in time would look like (like langue for Saussure, the system of language defined by phonematic, syntactic and semantic rules, in contrast to parole, the active use of langue in speech). It is important that in order for the symbolic order to be fully realized, we must in our speech bear witness to its authority (if only by acting as if we accept it) and submit to its laws. Where Lacan calls speech dominated by the imaginary, feint, he calls speech dominated by the symbolic, fides. The two are typically blended together which is a bearer of much of our self deception. As far as consciousness goes, one may suppose oneself to be simply acting straightforwardly according to what the rules and practices of some part of the symbolic order recognize. But this speech may actually be eroticized, be a feint designed to purloin the letter in the situation in behalf of one's desire. On the other hand, speech and relations with others which we feel to be most charged with our desire, may be strongly dominated by submission to the symbolic order.
The real register of speech, in, roughly, the Lacan of the mid-fifties to the mid-sixties, is speech which, perhaps very painfully, manifests or occasionally recognizes the split between the imaginary and the symbolic. That is, it is speech which more or less mutely registers the illusions of both the symbolic and the imaginary for giving meaning to human being. It is speech which in some way registers a deeper sense of what human freedom and unfreedom is than what is projected by symbolic and imaginary illusions of freedom. Thus, the real is not what we take for common or even scientific reality. Reality is a symbolic construction. Within the register of the real, there is access to the real power of the signifier which extends to matters or questions of significance which lie beyond the consideration of reality--though which affect what reality is considered to be.