The Mayo Clinic provides an official definition of a traumatic brain injury:
Traumatic brain injuries are injuries that impact our brains; some result in concussions and manageable symptoms, but others can result in disability or even death
TBIs often occur due to jolts or blows that impact the head
Sometimes, a trauma causes brain tissue penetration (like in the case of a shattered piece of skull damaging the brain); this can also lead to a TBI.
Mild TBIs usually impact our brain cells temporarily
Moderate or severe TBIs may lead to long-term bleeding, bruising, and torn tissue