A "concussion" is the historical term representing low-velocity injuries that cause brain "shaking." (1)(the word, in fact, is derived from the Latin concutere, meaning to shake violently) It is a subset of mTBI (mild traumatic brain injury)  (2), a term with which it is often used interchangeably, especially in the United States (1).
In layperson's terms, a concussion results from trauma (usally but not always a blow to the head) which causes the brain - a jellylike structure which is normally protected from collisions with the skull by a tough, fluid-filled membrane - to collide with the skull.
A concussion causes temporary metabolic changes in brain function (3) which one expert likens a concussion to a break in the local cables in a television network that results in lengthy re-routing of the call, so that when it finally gets through, it's delayed and full of static.
While no two concussions are exactly the same, a concussion is defined under the most recent international consensus statement on concussion in sport (1) as follows:
Concussion is a brain injury and is defined as a complex pathophysiological process affecting the brain, induced by biomechanical forces. Several common features that incorporate clinical, pathologic and biomechanical injury constructs that may be utilized in defining the nature of a concussion head injury include:
1. Direct blow to head not required. A concussion may be caused by either by a direct blow to the head, face, neck, or elsewhere on the body with a force transmitted to the head.
2. Rapid onset and gradual resolution of symptoms. A concussion typically results in the rapid onset of short-lived impairment of neurological functions that resolve spontaneously. However, in some cases, symptoms and signs  may evolve of a number of minutes to hours.
4. Loss of consciousness not required. 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 follow a sequential course. However, it is important to note that in some cases symptoms may be prolonged.
1. McCrory P, et al. Consensus statement on concussion in sport: the 4th International Conference on Concussion in Sport held in Zurich, November 2012. Br J Sports Med 2013;47:250-258.
2. Dematteo CA, Hanna SE, Mahoney WJ. et al. My child doesn't have a brain injury, he only has a concussion. Pediatrics 2010;125(2):327-334.
3. Giza CC, Hovda DA, The Neurometabolic Cascade of Concussion. J. Ath Train 2001;36(3):228-235.
Posted March 19, 2013