What is "black chamber"

n. (context historical cryptography informal English) A government or royal court's cryptanalyst department.

The Black Chamber, also known as The Cipher Bureau, was the United States' first peacetime cryptanalytic organization, and a forerunner of the National Security Agency. The only prior codes and cypher organizations maintained by the US government had been some intermittent, and always abandoned, attempts by Armed Forces branches prior to World War I.