Concussion

Medical condition

A concussion, also known as a mild traumatic brain injury (mTBI), is a head injury that temporarily affects brain functioning. Symptoms may include headache, dizziness, difficulty with thinking and concentration, sleep disturbances, a brief period of memory loss, brief loss of consciousness, problems with balance, nausea, blurred vision, and mood changes. Concussion should be suspected if a person hits their head and experiences any of the symptoms of concussion.

From Wikipedia, under CC BY-SA. More on occurri.