Neurodegeneration is the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases – including amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, fatal familial insomnia, and Huntington's disease – occur as a result of neurodegenerative processes.

Neurodegeneration is a complex multifactorial process that causes neuronal death in brain and spinal cord, resulting in brain and spinal cord damage and dysfunction. Neurodegeneration is accompanied by oxidative stress, axonal transport deficits, protein oligomerization, aggregation, calcium deregulation, mitochondrial dysfunction, abnormal neuron–glial interactions, neuroinflammation, DNA damage, and aberrant RNA processing. Neurodegeneration occurs in neurotraumatic, neurodegenerative, and neuropsychiatric diseases. These diseases are accompanied by structural, neurochemical, and electrophysiological abnormalities in the brain, spinal cord, and nerves causing paralysis, muscle weakness, poor coordination, seizures, confusion, and pain. Common neurotraumatic diseases include strokes, TBI, SCI, and CTE. In ischemic and traumatic brain and spinal cord injuries, neurons degenerate rapidly (in minutes to hours) because of the sudden lack of oxygen, a quick drop in ATP, and alteration in ion homeostasis.