A head injury might be a knock on a football field or the sudden impact of a blow. If someone falls and their head, accidentally, hits the pavement, then a severely disabling head injury can result. It’s not the head that’s damaged, it’s the brain inside.
Think of the brain as a jelly inside a hard canister. When the head shakes the brain doesn’t have any protection keeping it stable. It smashes from side to side inside the skull. Inevitably, it gets badly damaged. Even if the jelly seems intact, the delicate neural pathways within are often destroyed and need to be rebuilt.
The Victorians have a good definition.
“Acquired brain injury is injury to the brain which results in deterioration in
cognitive, physical, emotional or independent functioning. ABI can occur as a
result of trauma, hypoxia, infection, tumour, substance abuse, degenerative
neurological diseases or stroke. These impairments to cognitive abilities or
physical functioning may be either temporary or permanent and cause partial or
total disability or psychosocial maladjustment.”