By “the public sphere” we mean first of all a realm of our social life in which something approaching public opinion can be formed. Access is guaranteed to all citizens. A portion of the public sphere comes into being in every conversation in which private individuals assemble to form a public body. They then behave neither like business or professional people transacting private affairs, nor like members of a constitutional order subject to the legal constraints of a state bureaucracy. Citizens behave as a public body when they confer in an unrestricted fashion — that is, with the guarantee of freedom of assembly and association and the freedom to express and publish their opinions — about matters of general interest. In a large public body this kind of communication requires specific means for transmitting information and influencing those who receive it. Today newspapers and magazines, radio and television are the media of the public sphere. We speak of the political public sphere in contrast, for instance, to the literary one, when public discussion deals with objects connected to the activity of the state. Although state authority is so to speak the executor of the political public sphere, it is not a part of it. […]
It is no coincidence that these concepts of the public sphere and public opinion arose for the first time only in the eighteenth century. They acquire their specific meaning from a concrete historical situation. It was at that time that the distinction of “opinion” from “opinion publique” and “public opinion” came about. Though mere opinions (cultural assumptions, normative attitudes, collective prejudices and values) seem to persist unchanged in their natural form as a kind of sediment of history, public opinion can by definition only come into existence when a reasoning public is presupposed. Public discussions about the exercise of political power which are both critical in intent and institutionally guaranteed have not always existed — they grew out of a specific phase of bourgeois society and could enter into the order of the bourgeois constitutional state only as a result of a particular constellation of interests.
There is no indication European society of the high middle ages possessed a public sphere as a unique realm distinct from the private sphere. […]
[From Jürgen Habermas, “The Public Sphere: An Encyclopedia Article (1964),” transl. by Sara Lennox and Frank Lennox, New German Critique, 3 (1974): 49-50.]