Traumatic Brain Injury, commonly referred to as a head injury, is a brain dysfunction caused by an outside force, usually a violent blow to the head.
From the CDC (Centers for Disease Control and Prevention):
Traumatic Brain Injury Overview
Traumatic brain injury (TBI) is a major cause of death and disability in the United States, contributing to about 30% of all injury deaths. Every day, 138 people in the United States die from injuries that include TBI. Those who survive a TBI can face effects lasting a few days to disabilities which may last the rest of their lives. Effects of TBI can include impaired thinking or memory, movement, sensation (e.g., vision or hearing), or emotional functioning (e.g., personality changes, depression). These issues not only affect individuals but can have lasting effects on families and communities.