Alarm reaction is a concept in and is the first stage of stress response. Developed by Hans Selye, GAS proposes that our bodies have a natural, automatic, and adaptive response to stress that is composed of three stages: alarm reaction, resistance, exhaustion. The sympathetic nervous system is responsible for this reaction to a stressful stimulus. The alarm reaction stage is the 'fight or flight' and is characterized by a release of adrenaline that causes increased heart rate, faster breathing, perspiration, and dilated pupils.
It's the feeling that you would get if you were walking down a trail and suddenly realized you were about to step on a big snake!