Migraine is a chronic disorder characterized by recurrent moderate to severe headaches often in association with a number of autonomic nervous system symptoms. The word derives from the Greek (hemikrania), 'pain on one side of the head', from - (hemi-), 'half', and (kranion), 'skull'. Typically the headache is unilateral (affecting one half of the head) and pulsating in nature, lasting from 2 to 72 hours. Associated symptoms may include nausea, vomiting, photophobia (increased sensitivity to light), phonophobia (increased sensitivity to sound) and the pain is generally aggravated by physical activity. Up to one-third of people with migraine headaches perceive an aura: a transient visual, sensory, language, or motor disturbance which signals that the headache will soon occur. Migraines are believed to be due to a mixture of environmental and genetic factors.