Glossary of Neurological Terms

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This page includes plain language definitions of commonly used medical terms related to the brain.



Agnosia is the inability to recognize and identify objects or persons using one or more of the senses. It is caused by damage to the brain from strokes, brain injuries, dementia, or other neurological conditions. The inability to recognize something familiar does not necessarily mean that the person has problems with vision or thinking ability. 


Allodynia is the term for when something that previously was not painful to a person (light touch, for example) now causes pain.


Anemia is a condition in which a person’s blood does not produce enough healthy red blood cells which provide oxygen to body tissues. This can cause weakness and fatigue.


Aphasia is the loss of ability to speak or understand speech, usually caused by damage to the brain.


Apraxia is the loss of the ability to perform skilled movements and gestures. For example, a person may no longer be able to wink, lick their lips, or complete the steps required to bathe or dress themselves.


Atrophy is the process of wasting away or deteriorating in cells, tissues, and organs.

Atrial Fibrillation

Atrial fibrillation is a rapid, irregular, weak beating of the left atrium or upper chamber of the heart. It can cause blood clots and is a major risk factor for ischemic stroke.

Autonomic Nervous System

The autonomic nervous system is the part of the nervous system that controls involuntary functions such as breathing and heartbeat.

Autosomal Recessive Disorders

Autosomal recessive disorders refers to disorders in which means both parents carry and pass on a copy of a defective gene to the affected person. 


The axon is the part of the neuron that carries signals from the body of the neuron to the axon terminal, where the signal travels to other cells.



Biomarker is a term used to refer to biological signs of disease found in blood, body fluids, and tissues. Biomarkers can help indicate risk of a disease, aid in diagnosis, and track progression.

Blood-brain barrier

The blood-brain barrier is a layer of tightly-packed cells that control the movement of substances between the blood and the fluid that surrounds the brain’s neurons. It filters what goes in and out of the brain, allowing water, oxygen, and some medications to reach the brain. The blood-brain barrier also blocks harmful substances like bacteria, viruses, or other toxins before they reach the brain.


Central Nervous System

The central nervous system is the brain and spinal cord. It controls thinking, movement, and emotion, as well as autonomic functions like heartbeat, breathing, and body temperature. 


Cephalocele is a condition that arises as the brain is developing in which part of the brain and central nervous system protrude outside of the skull.


A term used to describe the principal part of the brain, the cerebrum.

Cerebral Atrophy

Cerebral atrophy includes neurons being injured and dying, connections between networks of neurons breaking down, and the gradual shrinking of brain regions.

Cerebral Hypoxia

Cerebral hypoxia is a condition in which the brain doesn’t get enough oxygen to function normally, even though blood flow is normal.

Cerebrospinal Fluid

Cerebrospinal fluid is a fluid that surrounds and fills the brain and spinal cord. It protects the brain, provides nourishment for cells, and removes waste.


Chorea is a movement disorder that causes sudden, unintended, and uncontrollable jerky movements of the muscles in the face, arms, or legs. It is a symptom of many conditions and diseases and is caused by overactivity of the chemical dopamine in the brain.


Clonus is a reflex response that includes involuntary and rhythmic muscle contractions. It is seen in some neurological conditions.


A contracture describes a shortening or tightening of the muscles or tendons around a person's joints. It usually results in a decreased range of motion in the joint.


A coma is a long, deep state of unconsciousness. People in a state of coma are alive but unable to move or respond to their surroundings.

Corpus Callosum

The corpus callosum is the bundle of nerve cells that connect the two hemispheres (halves) of the brain.



Dendrites are the part of the neuron that receive signals from other nerve cells.


Dyskinesia refers to involuntary movements, such as twisting and writhing. Tics, shaking, and tremor can also occur. 


Dysautonomia happens when there is a problem in the autonomic nervous system, which controls involuntary functions like breathing, heartbeat, and blood pressure which normally happen without thinking.


Dysgraphia is a term used to describe a variety of challenges and disabilities related to writing.


Dyslexia is a learning disorder that includes difficulty reading, particularly caused by challenges relating speech sounds to letters and words.


Dysphagia is the medical term for difficulty swallowing.


Dystonia is a medical term for involuntary muscle contractions or flexing. It can cause slow repetitive movements or abnormal postures that can be painful.



Encephalitis is inflammation of the brain.


Encephalopathy is a decrease in blood flow or oxygen to the brain.


Enzymes are proteins that help speed up chemical reactions in the body (called metabolism).



Fasciculations is the medical term for twitching muscles.



Hydromyelia is an abnormal widening of the spinal cord’s central canal in which cerebrospinal fluid can build up and damage nerve cells and their connections.


Hypersomnia is a medical term for excessive tiredness or sleepiness. It is different from feeling tired from lack of sleep at night and can result from an injury or problem in the brain.


Hypertonia is a medical term for too much muscle tone. It can make arms or legs stiff and difficult to move and often happens when part of the brain or spinal cord is damaged.


Hypotonia is a medical term for decreased muscle tone and can happen when the brain, spinal cord, nerves, or muscles are damaged.



Immunoglobulins are natural antibodies produced by the immune system.


Inflammation is a condition in which tissues, organs, or other parts of the body become swollen, hot, or painful. Inflammation can be a reaction to an infection or injury. Inflammation in the brain or spinal cord can cause other problems.

Intravenous immunoglobulin

Intravenous immunoglobulin (IVIg) is a common treatment for peripheral neuropathy caused by autoantibodies.


Learning Disabilities

Learning disabilities are disorders that affect a person’s ability to understand, or use spoken or written language, do mathematical calculations, coordinate movements, or direct attention. Learning disabilities occur in very young children but may not be identified until the child reaches school age. The most common treatment is special education and assistive devices.


A lesion is an area of abnormal or damaged tissue found inside or outside of the body. It can be caused by injury, infection, or disease.


Lipidoses (also known as lipid storage disease or disorders) are a group of inherited metabolic disorders in which harmful amounts of lipids build up in the body. 


Lipids are fatty materials in the body that include oils, fatty acids, waxes, and steroids (such as cholesterol and estrogen). They make up part of the cell membranes that help control what enters and leaves cells and help with storing energy, absorbing vitamins, and making hormones.



Meninges refers to the three layers of the membranes that protect the brain and spinal cord. The three layers are the pia mater (innermost), arachnoid (middle), and dura mater (tough outer layer).


Metabolism is the process by which the body processes food and other substances into a form that can be used to create energy. In the metabolic process, a substance can be broken down, absorbed, and used.

Multi-infarct Dementia

Multi-infarct dementia is a common cause of memory loss in older people and is caused by multiple strokes which damage brain tissue.


Myelin is a fatty molecule that provides insulation for the axon and helps nerve signals travel faster and farther.

Myelin Sheath

The myelin sheath is the insulating layer surrounding the axon, the message sender of the nerve cell.


Myelitis is an inflammation of the spinal cord. It can damage the myelin that covers the fibers of the nerve cell.


Myotonia is an impairment in a person’s ability to relax a muscle or group of muscles.


Nervous System

The nervous system includes the brain, spinal cord, and nerves throughout the body. It sends messages to and from the brain to the rest of the body.


Neurons, also called nerve cells, are the primary communication cells that send messages from the brain all over the body.


Neuropathy is an umbrella term that refers to damage or disease in the nerves, which can numbness or weakness.


Neurosarcoidosis is a chronic inflammatory disorder in the nervous system. Sarcoidosis primarily affects the lungs but can impact almost every other organ and system in the body.


Neurotransmitters are chemicals that send messages between and among neurons and other cells. They can be turn on actions (excitatory), turn off actions (inhibitory), or change actions (modulatory) that neurons can take.


Neurotoxicity happens when exposure to toxic substances affects the normal activity of the nervous system.


Orthostatic Hypotension

Orthostatic hypotension is a medical term that refers to a sudden drop in blood pressure when a person stands up.



Paralysis is an inability to move parts of the body. It may be accompanied by a loss of sensation in that part of the body.


Paresthesia refers to a burning, itching, tingling, or prickling sensation that is usually felt in the hands, arms, legs, or feet.

Peripheral Nervous System

The peripheral nervous system refers to the nerves that branch out from the brain and spinal cord to reach all other parts of the body.

Pinched Nerve

A pinched nerve is an informal term and not a true medical term. It is used to describe one type of damage or injury to a nerve or set of nerves. The injury may result from compression, constriction, or stretching.


Plasmapheresis is a procedure in which blood is removed, immune system cells and antibodies are taken out, and the blood is separated into plasma and blood cells. The blood is then returned to the body. 


Platelets are cell fragments in the blood that form clots to stop or prevent bleeding.


Prosopagnosia (also known as face blindness or facial agnosia) is a neurological disorder characterized by the inability to recognize faces. 



Rigidity is the medical term for involuntary muscle stiffness. It’s a form of hypertonia.



Spasticity is a medical term for an unexpected increase in muscle tone or stiffness which can interfere with movement and speech and cause discomfort or pain.


The synapse is a gap between two nerve cells through which signals pass via a neurotransmitter.


Syncope is a medical term for fainting or “passing out,” wherein a person becomes unconscious, goes limp, and quickly recovers.


Tardive Dyskinesia

Tardive dyskinesia causes repetitive, involuntary movements, such as grimacing and eye blinking. It is often caused by long-term use of antipsychotic medications.

Transcutaneous Electrical Nerve Stimulation

Transcutaneous electrical nerve stimulation (TENS) is a noninvasive intervention which involves attaching electrodes to the skin at the site of pain or near associated nerves and administering a gentle electrical current.


Tremor is a medical term for shaking in the hands, arms, legs, vocal cords, or other parts of the body.



Whiplash, also called neck sprain or neck strain, is a soft tissue injury to the neck, usually caused by sudden, fast movement.


White Matter

White matter is brain tissue made up of bundles of nerve fibers (axons) covered and protected by myelin. Myelin gives white matter its color. White matter is found in deep brain tissues and helps exchange information and facilitate communication between different areas of the brain.


Learn More About Disorders
Visit the disorders pages to learn about common neurological terms.

Brain Basics and Publications
Learn more about how the brain works and find printed materials on neurological disorders in the NINDS publications catalog.