(covers information from several alternate timelines)
A concussion is a common injury characterized by a loss of humanoid brain function as a result of a trauma. Whenever a humanoid feels dazed or is knocked unconscious after a blow to the head, he or she has had a concussion.
In 2152, Commander Charles Tucker III suffered a mild concussion when Shuttlepod 1 was damaged during the destruction of Paraagan II, but Doctor Phlox believed that he would be fine. (ENT: "Shockwave")
That same year, Ensign Hoshi Sato suffered a severe concussion when the Enterprise NX-01 was struck by a Romulan orbital mine. She eventually made a complete recovery, but was eager to return to her post, even before Doctor Phlox had finished treating her. (ENT: "Minefield")
In 2370, after shifting into an alternate quantum reality, Lieutenant Worf had suffered a concussion after being hit over the head in a Bat'leth competition that he had won in the prime reality. Doctor Beverly Crusher recommended vertazine to treat it. (TNG: "Parallels")