From the glossary:
This is the speaking personality, the speaking consciousness. A voice always has a will or desire behind it, its own timbre and overtones. Single-voiced discourse is the dream of poets; double-voiced discourse the realm of the novel. At several points Bakhtin illustrates the difference between these categories by moving language-units from one plane to the other — for example, shifting a trope from the plane of poetry to the plane of prose: both poetic and prose tropes are ambiguous [literally "double-meaninged"] but a poetic trope, while meaning more than one thing, is always only single-voiced. Prose tropes by contrast always contain more than one voice, and are therefore dialogised.
The base condition governing the operation of meaning in any utterance. It is that which insures the primacy of context over text. At any given time, in any given place, there will be a set of conditions — social, historical, meteorological, physiological — that will insure that a word uttered in that place and at that time will have a meaning different than it would have under any other conditions; all utterances are heteroglot in that they are functions of a matrix of forces practically impossible to recoup, and therefore impossible to resolve. Heteroglossia is as close a conceptualisation as is possible of that locus where centripetal and centrifugal forces collide; as such, it is that which a systematic linguistics must always suppress.
Dialogism is the characteristic epistemological mode of a world dominated by heteroglossia. Everything means, is understood, as a part of a greater whole-there is a constant interaction between meanings, all of which have the potential of conditioning others. Which will affect the other, how it will do so and in what degree is what is actually settled at the moment of utterance. This dialogic imperative, mandated by the pre-existence of the language world relative to any of its current inhabitants, insures that there can be no actual monologue. One may, like a primitive tribe that knows only its own limits, be deluded into thinking there is one language, or one may, as grammarians, certain political figures and normative framers of "literary languages" do, seek in a sophisticated way to achieve a unitary language. In both cases the unitariness is relative to the overpowering force of heteroglossia, and thus dialogism.