Fight or Flight: The reaction that occurs in the body when faced by a sudden, unexpected threat or source of stress. The name of this reaction comes from the fact that an animal experiencing this reaction almost immediately decides to fight or to run. In this reaction there is a sudden release of the hormones epinephrine and norepinephrine, which increase blood flow to the muscles, increase arterial blood pressure, improve muscle strength and mental ability, as well as increase blood glucose Concentration. Through these actions the body prepares for a confrontation or a fast escape.

Tactical Definition of Fight or Flight:  It's about to hit the fan! You have to decide if it is best to fight or run like hell. Example one: You live on the coast and a Hurricane is expected to hit land in a short while. Do you stay and fight or run like hell.  Our choice is to run ( drive actually) like the devil was on my heels.  You can't beat the hurricane at it's own game. Example Two: While on a short road trip you stop at a fast food restaurant for a break. While enjoying the fried grease two pukes attempt to rob the place and decide to have a stand off with the local PD. Do you fight or flight? I would again run like hell if possible, if not fight when the chance presented itself.