An injury was damage to the body caused by external force.
An injury could be the cause of regeneration. (TV: Logopolis, Time and the Rani, Doctor Who, Utopia, Let's Kill Hitler, Hell Bent, The Doctor Falls, Twice Upon a Time, PROSE: Interference - Book Two)
When the Twelfth Doctor broke his toe, Clara Oswald suggested that he regenerate to heal the injury, but he berated the idea as a waste. (PROSE: The Blood Cell)
When Chula nanogenes found a dying child named Jamie in the London Blitz, they took his injuries as a template for all humans. When they went on to spread the Empty Child virus, those same injuries — severe head trauma on the left side, partial collapse of the chest cavity of the right, and a gash on the back of the right hand — presented themselves in everyone who caught it. (TV: The Empty Child / The Doctor Dances)
Humans also had an innate ability to heal from injuries, on a far smaller scale. (AUDIO: The Invention of Death)