Answer:
The term "trias politica" or "separation of powers" was coined by Charles-Louis de Secondat, baron de La Brède et de Montesquieu, an 18th century French social and political philosopher.
Or you could put:
Separation of powers is a political doctrine originating in the writings of Charles de Secondat, Baron de Montesquieu in The Spirit of the Laws, in which he argued for a constitutional government with three separate branches, each of which would have defined abilities to check the powers of the others.