Concussion is an injury to the brain caused by being hit on the head, as often happens in motor vehicle collisions, fights or contact sports such as football. It is the most common form of traumatic brain injury. Related terms include mild brain injury, mild traumatic brain injury (MTBI), mild head injury (MHI), and minor head trauma. Patients with severe concussion are occasionally monitored in hospital. The main reason for this is the risk of bleeding in the brain, which can be life-threatening. 