Wikipedia defines it thusly:
Hypervigilance is an enhanced state of sensory sensitivity accompanied by an exaggerated intensity of behaviors whose purpose is to detect threats. Hypervigilance is also accompanied by a state of increased anxiety which can cause exhaustion. Other symptoms include: abnormally increased arousal, a high responsiveness to stimuli, and a constant scanning of the environment for threats. Hypervigilance can be a symptom of posttraumatic stress disorder and various types of anxiety disorder. It is distinguished from paranoia. Paranoid states, such as those in schizophrenia, can seem superficially similar, but are characteristically different.
I define it as freaking the fuck out at a raised voice, a passing car, a squeaking door.
It is bursting into tears because I am frightened ALL THE FUCKING TIME.
It is feeling my heartbeat in my ears and in my arms and my ARSE… every part of my body is on hyper alert.
It is not being able to breathe and wanting to hide and never leave my house because I know the noises of my home and where to sit to scan my environment without getting motherfucking whiplash.
It is by the end of the day I am so physically exhausted from trying to hold my shit together that I fall into a deep sleep only to be awakened by the DESPERATE FEAR that something is going to happen in the middle of the night.
And this – despite everything else, and I mean EVERYTHING ELSE – is the thing that may just make me lose my fucking mind.