A state of extreme alert to potential danger. This can occur in a number of different mental health disorders, but is most closely tied to PTSD and even in other diagnoses seems to stem from a traumatic experience and resulting in the perceived need to be on heightened alert for danger.
Hypervigilance is maintained and serves to reinforce the fight or flight system to be ready to deploy at any moment. So even though this alertness is intended to help the person prevent harm from dangerous people or situations, it often leads to chronic feelings of tiredness and exhaustion.
Being on alert to interpersonal danger can also lead to problems in relationships with family, friends and co-workers. Learning a healthy level of self-awareness and preparedness is difficult because it can be hard for the person to see that their reaction is extreme. Therapy and education can be quite helpful.