fourth estate (n.)
"the press," by 1824, and especially from 1831, British English. For the other three, see estate. Earlier the term had been applied in various senses that did not stick, including "the mob" (1752), "the lawyers" (1825). The extension to the press is perhaps an outgrowth of the former.
Hence, through the light of letters and the liberty of the press, public opinion has risen to the rank of a fourth estate in our constitution; in times of quiet and order, silent and still, but in the collisions of the different branches of our government, deciding as an umpire with unbounded authority. ["Memoir of James Currie, M.D.," 1831]
[Newspapers] began to assume some degree of political importance, during the civil wars of the seventeenth century, in England; but it is not until within the last fifty years that they have become, — as they are now justly styled, — a Fourth Estate, exercising a more powerful influence on the public affairs of the countries in which they are permitted to circulate freely, than the other three put together. [Alexander H. Everett, "Address to the Phi Beta Kappa Society of Bowdoin College," 1834]