Post-traumatic stress disorder was a psychological condition caused by exposure to an extremely traumatic event. Characteristic symptoms included anxiety and flashbacks.
Marla Gilmore, a crew member aboard the USS Equinox, suffered from post-traumatic stress syndrome after enduring repeated attacks by nucleogenic lifeforms. This caused her to request reassignment to a repair team aboard the USS Voyager after the two ships met in late 2375. (VOY: "Equinox")
Neelix experienced a condition neurochemically identical to post-traumatic stress disorder in 2376, due to the memories of the Nakan massacre placed into his mind by a memorial monument on Tarakis. When Naomi Wildman accidentally burned herself in his kitchen, Neelix flashed back to the massacre and, brandishing a phaser, barricaded them inside the mess hall. (VOY: "Memorial")