-“A concussion is a type of traumatic brain injury—or TBI—caused by a bump, blow, or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth. This sudden movement can cause the brain to bounce around or twist in the skull, creating chemical changes in the brain and sometimes stretching and damaging brain cells.” CDC
-A concussion is a change/disturbance in brain function caused by direct or indirect force to the head.
-A concussion is a change in how the brain functions. It is a functional injury, not a structural injury to the brain.
-Loss of consciousness is not necessary for it to be a concussion.
-Direct impact to the head is not required.
A headache is the most common symptom of a concussion. Signs and symptoms are usually non-specific.
References
https://www.cdc.gov/headsup/basics/concussion_whatis.html
http://www.aafp.org/afp/2012/0115/p123.html