What is Estates Of The Realm?

Legal Definition
The estates of the realm were the broad orders of social hierarchy used in Christendom (Christian Europe) from the medieval period to early modern Europe. Different systems for dividing society members into estates developed and evolved over time.

The best known system is the French Ancien Régime (Old Regime), a three-estate system used until the French Revolution (1789–1799). This system was made up of clergy (the First Estate), nobility (the Second Estate), and commoners (the Third Estate). In some regions, notably Scandinavia and Russia, burghers (the urban merchant class) and rural commoners were split into separate estates, creating a four-estate system with rural commoners ranking the lowest as the Fourth Estate. In England, a two-estate system evolved that combined nobility and bishops into one lordly estate with "commons" as the second estate. This system produced the two houses of parliament, the House of Commons and the House of Lords. In southern Germany, a three-estate system of nobility (princes and high clergy), ritters (knights), and burghers was used.

Today the term "Fourth Estate" usually refers to forces outside the established power structure (evoking medieval three-estate systems), most commonly in reference to the independent press or media. Historically, in Northern and Eastern Europe, the Fourth Estate meant rural commoners.
-- Wikipedia
Legal Definition
The lords spiritual, the lords temporal, and the commons of Great Britain. 1 Bl. Comm. 153. Sometimes called the "three estates."
-- Black's Law Dictionary
Legal Definition
The lords spiritual and temporal and the commons.
-- Ballentine's Law Dictionary