HD Glossary

Note: Some words can have differing definitions depending on the context in which they are used. The definitions presented here are the ones that best apply to the words as they are used. HDSA is grateful to the Stanford Hopes website for their contribution and maintenance of this glossary.



  • Obsessive-Compulsive Disorder - A disorder characterized by recurrent thoughts or behaviors (e.g., excessive hand washing or repeated doubt about having locked a door) that are severe enough to be time consuming or cause significant impairment in social functioning.
  • occupational therapy - a type of therapy that teaches (or re-teaches) individuals the skills they would need to lead balanced lives and perform necessary functions for everyday living, and may either be skills that pertain to the home or to work.
  • oligomenorrhea - Infrequent or very light menstruation; risk factor for osteoporosis.
  • oligopeptide - Several amino acids linked together.
  • omega-3 fatty acid - A type of fatty acid that our body derives from food. Omega-3 fatty acids have anti-inflammatory properties due to their ability to convert into anti-inflammatory prostaglandins.
  • omega-6 fatty acid - Essential fatty acids that use the same enzymes as omega-3 fatty acids to produce their final products.
  • operationalize - The process of converting concepts into specific observable behaviors that a researcher can measure.
  • optic nerve - A collection of nerve cells that project visual information from the eyes to the lateral geniculate nucleus.
  • optic tract - Fibers of the optic nerve which project visual information mainly to the lateral geniculate body.
  • optimization - The process by which a chemical compound is modified by chemical or biological means to make it interact more effectively with the biological target.
  • optimal dosage - The dosage that would be best for the patient population at large. It confers the most benefit with the least amount of side effects.
  • organelle - A membrane-bound compartment in a cell with a specialized function.
  • organism - Any living thing.
  • osteoporosis - A disease in which the bones are weakened by reduced calcium absorption. Long-term use of glucocorticoids can result in this condition.
  • oxidation - The process by which a molecule or atom loses an electron. Oxidation of cell components by free radicals can accelerate cell death and plays a role in the HD disease process.
  • oxidative damage - The harmful condition that occurs when there is an excess of free radicals, a decrease in antioxidant levels, or both.
  • oxidative phosphorylation - Refers to the coupling of the electron transport chain to ATP synthesis via the proton gradient and ATP synthase. This occurs primarily in the presence of oxygen.
  • oxidative stress - See oxidative damage.
  • oxygen - An element active in physiological processes and crucial to brain functioning.