Traumatic brain injury (TBI), also called intracranial injury or simply head injury, occurs when physical trauma causes brain damage. TBI can result from a closed head injury or a penetrating head injury and is one of two subsets of acquired brain injury (ABI). The other subset is non-traumatic brain injury, or injuries that do not involve external mechanical force (e.g. stroke, meningitis, anoxia). Parts of the brain that can be damaged include the cerebral hemispheres, cerebellum, and brain stem.

TBI is usually classified as mild, moderate, or severe, depending on the extent of loss of consciousness, posttraumatic amnesia, and Glasgow coma scale following the injury. The severity of the deficits caused by a TBI is related to its classification but not necessarily determined by it. Severe TBI sometimes produces less impairment than mild or moderate TBI. TBI can cause a host of physical, cognitive, emotional, and social effects. Outcome can be anything from complete recovery to permanent disability or death.