Discourse is unified speaking or text that exceeds one sentence. The order of discourse is of the same order as that of speaking, enlightening and knowledge:
[It] is of the very nature of ... language to be knowledge from its very first word. Speaking, enlightening, and knowing are, in the strict sense of the term, of the same order. (Foucault, The Order of Things)
Discourse is definitive, not representational: discourse defines and creates reality, it does not represent external reality.
Discourse is power: discourse is knowledge; knowledge is power; discourse is power.
[Discourse is] ways of constituting knowledge, together with the social practices, forms of subjectivity and power relations which inhere in such knowledges and relations between them. Discourses are more than ways of thinking and producing meaning [through speech or text]. They constitute the 'nature' of the body, unconscious and conscious mind and emotional life of the subjects they seek to govern. (Pinkus, 1996, on Weedon, 1987).
Discourse is the mulitfaceted flow of power through multiple layers of participants as in Foucault's illustration of the painter looking at the painting while you are looking at the frame-back of the painting, at the painter and at the background while someone else is looking at you.
"[T]his dotted line reaches out to us ineluctably, and links us to the representation of the picture. In appearance, this locus is a simple one; a matter of pure reciprocity: we are looking at a picture in which the painter is in turn looking out at us. ... in this precise but neutral place, the observer and the observed take part in a ceaseless exchange."(Foucault, The Order of Things)
Discourse is the linking structure of meaning uniting langage and knowledge within Literature, a link that is now enigmatic (mysterious, unclear) in Literature since the "reciprocal kinship between knowledge and language" was modified in the nineteenth century (1800s).
[There existed a linking] reciprocal kinship between knowledge and language. The nineteenth century was to dissolve that link, and to leave behind it, in confrontation, a knowledge closed in upon itself and a pure language that had become, in nature and function, enigmatic - something that has been called, since that time, Literature." (Foucault, The Order of Things)
One example from a novel of this connection between discourse and Literature is taken from Walpole's The Cathedral. Walpole's representation of the Cathedral is enigmatic in that (1) it appears to be a character and (2) it seems a villainous character instead of a noble one. Archdeacon Brandon, who is also enigmatically drawn and represented as both villain and hero, begins to feel the Cathedral closing in on him after the failures and disappointments he has undergone:
the Cathedral seemed so close. It was not close really, ... but now Brandon had the strange fancy that it had drawn closer during these last weeks, and was leaning forward with its ear to his house, listening just as a man might! [...] [Joan] rose very softly ... and looked out. ... The Cathedral seemed to be very close to the house. (Walpole, The Cathedral)