In France under the ancien régime, the Estates of the realm were the three divisions of the Estates-General. The First Estate was the clergy, the Second was nobility, the Third was the commoners. In theory, all commoners were part of the Third Estate. In practice, during the period where there was a division of France into estates, of the commoners only the bourgeoisie were represented. The division of the French realm into estates was abolished during the first months of the French Revolution.
The use of this term is not exclusive to France (although that is by far its most common usage). In the United Kingdom, an analogous division exists to this day, although with attenuated significance, between Lords Temporal, Lords Spiritual, and Commons.
Note one contrast between the French and British systems: the lower clergy in France were part of the First Estate, but in Britain they are commoners.
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