Operations security
Operations security (OPSEC) or operational security is a process that identifies critical information to determine whether friendly actions can be observed by enemy intelligence, determines if information obtained by adversaries could be interpreted to be useful to them, and then executes selected measures that eliminate or reduce adversary exploitation of friendly critical information. The term "operations security" was coined by the United States military during the Vietnam War. History

Vietnam 
In 1966, United States Admiral Ulysses Sharp established a multidisciplinary security team to investigate the failure of certain combat operations during the Vietnam War. This operation was dubbed Operation Purple Dragon, and included personnel from the National Security Agency and the Department of Defense. When the operation concluded, the Purple Dragon team codified their recommendations. They called the process "Operations Security" in order to distinguish the process from existing processes and ensure continued inter-agency support. NSDD 298 
In 1988, President Ronald Reagan signed National Security Decision Directive (NSDD) 298. This document established the National Operations Security Program and named the Director of the National Security Agency as the executive agent for inter-agency OPSEC support. This document also established the Interagency OPSEC Support Staff (IOSS). Private-sector application 
The private sector has also adopted OPSEC as a defensive measure against competitive intelligence collection efforts. See also
For Official Use Only – FOUO
Information security
Intelligence cycle security
Security
Security Culture
Sensitive but unclassified – SBU
Controlled Unclassified Information - CUI
Social engineering

References

Further reading 

 National Security Decision Directive 298
Purple Dragon: The Origin & Development of the United States OPSEC Program, NSA, 1993. Operations Security (JP 3-13.3) PDF U.S. DoD Operations Security Doctrine. External links

U.S. Government OPSEC site
Operations Security Professionals Association
How to Conduct an OPSEC Assessment

Information operations and warfare
Espionage
Computer security