DEFCON

DEFCON is an alert posture used by the United States armed forces. The current DEFCON level in real time is Level 4. It is mentioned many times in Call of Duty: Black Ops' campaign in the mission briefings as a sign of warning with Nova-6 before released.

In-Game
The first DEFCON level in Black Ops' single-player is at DEFCON 4 but is changed quickly to L3 in WMD. Before Rebirth, DEFCON is moved to L2, which means preperations for war. Finally, DEFCON is moved down to its final level, L1, meaning an all-out war before a US team comprised of Mason, Hudson and Weaver stopped the Nova-6 threat.

DEFCON is seen in the Zombies map "Five", where players will have to switch the level to gain rewards.