Estates of the realm. The powers that have the administration of affairs in their hands. The three estates of our own realm are the Lords Spiritual, the Lords Temporal, and the Commons; popularly speaking, the public press is termed the fourth estate. It is a great mistake to call the three estates of England the Sovereign, the Lords, and the Commons, as many do. The word means that on which the realm stands. (Latin, sto, to stand.) (See Fourth Estate.)
“Herod ... made a supper to his ... chief estates.”—Mark vi. 21.
“The king and the three estates of the realm assembled in parliament.” —Collect for Nov. 5.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Estates from Fact Monster: