Traumatic Head Injury (THI)a	Concussion-associated THI, with 1 or more occasions during which all 3 postconcussive symptoms (loss of consciousness, posttraumatic transient amnesia, and any neurologic deficits) occurred.
Traumatic Brain Injury (TBI)b	TBI entails damage to the brain resulting from an external force (e.g., brain laceration, intracranial hematoma, intracranial hypertension, hypoxia, anemia, contusion, metabolic anomalies, shearing of the fibers of nerves, and contrecoup injuries). It is not due to a degenerative or congenital insult to the brain. It may lead to temporary or permanent impairment of cognitive, physical, and psychosocial functions, with associated altered or reduced state of consciousness.
Post Concussive Syndrome (PCS)b	A syndrome that includes fatigue, sensitivity to light or sound, headache, dizziness, sleep disturbance, and concentration problems. The symptoms of PCS overlap with some of the symptoms of PTSD.

Notes: Definitions vary from source to source.

^aDefinition is from Mollica RF, Lyoo IK, Chernoff MC, Bui HX, Lavelle J, Yoon SJ, et al. Brain structural abnormalities and mental health sequelae in South Vietnamese ex-political detainees who survived traumatic head injury and torture. Arch Gen Psychiatry. 2009;66(11):1221-1232.

^bDefinition is from Bryant R. Posttraumatic stress disorder vs. traumatic brain injury. Dialogues in Clinical Neuroscience. 2011;13(3):251-262.