What is a hit on the head called?

A hit on the head is commonly referred to as a "head injury." Depending on the severity and nature of the impact, it can also be classified as a "concussion" if it results in a temporary disturbance of brain function. Other potential terms include "traumatic brain injury" (TBI) for more serious injuries or simply "blow to the head." Always seek medical attention if a head injury occurs, especially if symptoms arise.