Discrepant criteria are utilized by various disciplines for the diagnosis of mild traumatic brain injury (TBI). This study evaluates 76 patients, all of whom were diagnosed as having sustained a mild TBI according to the diagnostic criteria set forth by the American Congress of Rehabilitation Medicine (ACRM); yet only 34% of these patients were classified as having a concussion according to DSM-IV. A unified definition is proposed which is comprised of grades: Type I for ACRM, Type III for DSM-IV, and Type II to bridge the two discrepant definitions. An examination of the patients, subdivided into the three types, revealed no significant differences for (1) number of subjective complaints, (2) neurocognitive performances, and (3) pre-existing emotional risk factors. Thus, the proposed gradation unifies the definitions across the heterogeneity of mild TBI. However, further research is indicated for their clinical validation.