Per the Palmerton Sports Medicine - St. Luke's Sports Medicine Concussion Management Guidelines:

A concussion is a brain injury and is defined as a complex pathophysiological process affecting the brain, induced by biochemical forces.Several common features that incorporate clinical, pathologic and biomechanical injury constructs that may be utilized in defining the nature of a concussive head injury include:- A concussion may be caused either by a direct blow to the head, face, neck or elsewhere on the body with an "impulsive" force transmitted to the head- A concussion typically results in the rapid onset of short-lived impairment of neurological function that resolves spontaneously and may occur with or without a loss of consciousness. However, in some cases, symptoms and signs may evolve over a number of minutes to hours.- A concussion may result in neuro-pathological changes, but the acute clinical symptoms largely reflect a functional disturbance rather than a structural injury and, as such, no abnormality is seen on standard structural neuroimaging studies.- A concussion results in a graded set of clinical symptoms that may or may not involve loss of consciousness. Resolution of the clinical and cognitive symptoms typically follows a sequential course. However, it is important to note that in some cases symptoms may be prolonged.Signs of concussionThe signs of a concussion include:- Appears dazed, stunned, or disoriented, demonstrates decreased alertness.- Forgets plays, or demonstrates short term memory difficulty.- Slurs words.- Exhibits difficulties with balance or coordination.- Answers questions slowly or inaccurately.- Exhibits seizures or vomiting.- Changes in level of consciousness.Symptoms of concussionThe symptoms of a concussion include headache; nausea; balance problems or dizziness; double vision or changes in vision; sensitivity to light or sound/noise; feeling sluggish or foggy; difficulty with concentration and short term memory; sleep disturbance; or irritability or changes in personality and behavior.