Post-traumatic stress disorder or syndrome, shortened as PTSD, was a psychological condition caused by exposure to an extremely traumatic event. Characteristic symptoms included anxiety and flashbacks. Patients might also go into shock, at which point they would be unable to function. During the mid-23rd century, Starfleet PTSD regulations required full-duty quarantine until treatment was possible. (DIS: "Into the Forest I Go", "Despite Yourself")
In the personnel file of Spock, it was noted that there was a medical log giving information about his diagnosis and treatment for this condition. (SNW: "Among the Lotus Eaters", display graphic)
Katrina Cornwell had experience treating people with PTSD. (DIS: "Into the Forest I Go")
Lieutenant Ash Tyler appeared to suffer from post traumatic stress disorder following his imprisonment and torture by the Klingon L'Rell. Tyler suffered from nightmares after his rescue. Additionally Tyler experienced intense flashbacks after encountering L'Rell on board the Sarcophagus. (DIS: "Into the Forest I Go")
In 2374, The Doctor speculated that Seven of Nine's hallucinations of the Borg and a raven might be caused by post-traumatic stress disorder, brought on by her assimilation. (VOY: "The Raven")
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")