Combat air patrol — An aircraft patrol provided over an objective area, the force protected, the critical area of a combat zone, or in an air defense area, for the purpose of intercepting and destroying hostile aircraft before they reach their targets. Also called CAP.
Joint Publication 1-02, "Department of Defense Dictionary of Military and Associated Terms." U.S. DTIC.
A straightforward phrase, consisting of each of its three components. Combat - units involved in CAP are there for the express purpose of engaging in combat if required. Air - this is an activity performed in the air, by aircraft. Patrol - the unit or units involved are to patrol a specified area or route for the purpose of intercepting enemy aircraft.
This activity is sometimes further identified depending on the type of objective that is being protected or pursued. For example, BARCAP stands for "Barrier Combat Air Patrol" - and is just that. It is a CAP intended to act as a barrier to prevent enemy aircraft from crossing it, rather than being tied to a specific force or objective on the ground. RESCAP stands for "Rescue Combat Air Patrol" - whenever an airborne rescue mission goes out into hostile territory with air-to-air threats, a CAP will likely be assigned to prevent enemy aircraft from interfering with the rescue mission.
General CAP is typically performed as a regular activity around high-profile targets. For example, U.S. Navy aircraft carriers, the centers of their battle groups, will maintain a Combat Air Patrol at regular distances from the carrier to prevent hostile aircraft from getting close enough to threaten the ship. After 9/11, the US DoD specified that a Combat Air Patrol would be instantiated above the CONUS in order to provide 'quick response' against similar attacks.