CI meaning in Military and Defence ?

Counterintelligence is an activity aimed at protecting an agency's intelligence program from an opposition's intelligence service. It includes gathering information and conducting activities to prevent espionage, sabotage, assassinations or other intelligence activities conducted by, for, or on behalf of foreign powers, organizations or persons.

Many countries will have multiple organisations focusing on a different aspect of counterintelligence, such as domestic, international, and counter-terrorism. Some states will formalise it as part of the police structure, such as the United States' Federal Bureau of Investigation. Others will establish independent bodies, such as the United Kingdom's MI5.