What do the initials "MI" refer to in MI5 and MI6?

The initials "MI" in MI5 and MI6 stand for Military Intelligence. This designation reflects the historical roots of these agencies, which were established to gather and analyze intelligence that was critical for military operations and national security. MI5, known as the Security Service, primarily focuses on domestic intelligence and protecting the UK from threats such as terrorism and espionage. On the other hand, MI6, or the Secret Intelligence Service, deals with foreign intelligence and operations abroad.

These agencies' origins trace back to the early 20th century, where the term Military Intelligence was commonly used to describe units responsible for gathering and assessing information related to military affairs. This historical context highlights their purpose and operational focus in the intelligence community, emphasizing the importance of military insights in both domestic and foreign security matters.