Abdominal migraine: a migraine, more common in children and adolescents, that is associated with cyclic vomiting (symptoms occur about once a month).

Abortive medications: medications used to stop the headache process and prevent symptoms of migraines, including pain, nausea, sound and light sensitivity, etc. They are most effective when used at the first sign of a migraine to stop the process which causes the headache pain.

Abscess: a localized collection of pus in tissues, organs, or confined spaces usually due to an infection.

Acupuncture: an ancient Chinese system of health care that involves the insertion of very fine, solid needles into certain points of the body. Acupuncture stimulates the body's ability to resist or overcome illnesses and conditions by correcting energy imbalances. Acupuncture also prompts the body to produce chemicals that decrease or eliminate painful sensations.

Acute: sudden; occurs quickly and generally, without warning.

Acute headaches: headaches that occur suddenly for the first time with symptoms that subside after a relatively short period of time. They are usually due to an illness, infection, cold, or fever.

Acute recurrent headaches: see migraines.

Adrenaline (epinephrine): the neurotransmitter of the adrenal gland that is secreted in moments of crisis. It stimulates the heart to beat faster and work harder, increases the flow of blood to the muscles, causes an increased alertness of mind, and produces other changes to prepare the body to meet an emergency. It is also a chemical messenger in the brain.

Analgesic: pain-relieving medication.

Analgesic-rebound headache: see rebound headache.

Aneurysm: a weak part of an artery that may bulge outward and occasionally rupture and bleed, leading to a condition called a subarachnoid hemorrhage which produces a severe headache and stiff neck, and can sometimes be fatal.

Anticonvulsant: a type of medication used to treat convulsive seizures, or epilepsy. Some of these types of medications are also used to prevent headaches, even when the headaches aren't associated with seizures.

Antidepressant: a type of medication used primarily to treat depression. Some of these medications have been useful in treating headaches, even when headaches aren't associated with depression.

Antiemetics: a class of medications used to treat nausea and/or vomiting.

Antihistamine: a drug that counteracts the action of histamine, an agent in the body that causes itching and flushing of the skin such as in an allergic reaction.

Anti-inflammatory: a type of medication used to decrease inflammation. This type of medication is most commonly used to treat the inflammation of arthritis and other inflammatory disorders, but can also be useful in reducing the pain of certain types of headaches.

Arnold-Chiari deformity: a congenital abnormality in which the back part of the brain (cerebellum) and brain stem protrude down into the spinal canal through the large hole at the base of the skull where the spinal cord passes. It may be associated with many other defects, including a certain form of spina bifida and can cause headaches.