The term "Fourth Estate" refers to the press, both in its explicit capacity of advocacy and in its implicit ability to frame political issues. The term goes back at least to Thomas Carlyle.
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2 Alternate Meaning
3 See also
4 External links
Primary meaning of the term
In Carlyle's On Heroes and Hero Worship (1841), he writes, "... does not... the parliamentary debate go on... in a far more comprehensive way, out of Parliament altogether? Burke said that there were three Estates in Parliament, but in the Reporters Gallery yonder, there sat a fourth Estate more important far than they all." 
This was not Carlyle's first use of the term. If, indeed, Burke did make the statement Carlyle attributes to him, Burke's remark may have been in the back of Carlyle's mind when he wrote in his French Revolution (1837) "A Fourth Estate, of Able Editors, springs up."  In this context, the other three "Estates" are those of the French States-General (Burke, as author of Reflections on the Revolution in France, could have had in mind precisely these three Estates, or the three referred to by Henry Fielding in the quotation below.)
The term Fourth Estate has also (relatively infrequently) been used to refer specifically to the proletariat as against the three recognized estates of the French ancien régime. This use is uncommon to the point of being potentially confusing.
Interestingly, an even earlier citation can be found for this use than for the one that now prevails: Henry Fielding, Covent Garden Journal (1752): "None of our political writers... take notice of any more than three estates, namely, Kings, Lords, and Commons... passing by in silence that very large and powerful body which form the fourth estate in this community... The Mob." (quoted at , which unfortunately has pop-ups.)