From Pluralpedia, the collaborative plurality dictionary
OriginPsychiatric term

A flashback is the experience of reliving a traumatic event: it can contain a sensory experience of complete immersion in the event, but not necessarily! This can be manifested by a sensory reliving (images, sounds, smells), but also physical sensations (pain, pressure), strong emotions felt during the trauma, or intrusive thoughts (very negative thought loop, mental regression). A flashback can also take a person back to the event so vividly that it feels like they are actually there physically. A flashback is not dangerous, but it can be up to as traumatic as the event itself were.