A war veteran describes sudden, unbidden moments where he is abruptly plunged back into the sights and sounds of combat, feeling for those moments as though it is happening again right now, complete with the original fear and physical tension. What is this?

This is a flashback: a sudden, intrusive memory bringing with it the cognitive and emotional quality of the original event, rather than a straightforward act of voluntary recall, which may lead to acting or feeling, at least momentarily, as though the traumatic event is happening again in the present, exactly as described here. It is not a flashbulb memory, which is a stable, richly detailed memory recalled voluntarily and with confidence, not a sudden, intrusive re-experiencing. It is not an ordinary intrusive recollection, since a flashback is distinguished from that by its vivid, re-experienced, emotionally immediate quality — this goes well beyond an unwelcome memory simply coming to mind. And it is not confabulation, which fills a gap caused by organic memory impairment with fabricated material, not a re-experienced memory of a genuine event.