I would have to say this is one of the better defininations of a migraine I have seen. It is somewhat lengthy but worth the read. Please go directly to the website for the references as I have ommitted them for space purposes. I have also edited some of the content. This was taken from Wikipedia.com: http://en.wikipedia.org/wiki/Migraine
"Migraine is a neurological disease that can cause a wide range of symptoms during an attack. The most commonly thought of symptom is headache. It is widespread in the population. In the U.S., 18% of women and 6% of men report having had at least one migraine episode in the previous year, with seriousness ranging from an annoyance to a life-threatening and/or daily experience. Periodic or unpredictable disability can cause poverty due to patients' inability to hold down a job. Overview Usually migraine causes episodes of severe or moderate headache (which is often one-sided and pulsating) lasting from several hours to three days, accompanied by gastrointestinal upsets, such as nausea and vomiting, and a heightened sensitivity to bright lights (photophobia) and noise (phonophobia).
Approximately one third of people who experience migraine get a preceding aura. The word migraine is French in origin and comes from the Greek hemicrania, as does the Old English term megrim. Literally, hemicrania means "half (the) head". Migraines' secondary characteristics are inconsistent.
Triggers precipitating a particular episode of migraine vary widely. The efficacy of the simplest treatment, applying warmth or coolness to the affected area of the head, varies between persons, sometimes worsening the migraine. A particular migraine rescue drug may sometimes work and sometimes not work in the same patient. Some migraine types don't have pain or may manifest symptoms in parts of the body other than the head. Available evidence suggests that migraine pain is one symptom of several to many disorders of the serotonergic control system, a dual hormone-neurotransmitter with numerous types of receptors. Two disorders — classic migraine with aura (MA, STG) and common migraine without aura (MO, STG) — have been shown to have a genetic factor. Studies on twins show that genes have a 60 to 65% influence on the development of migraine (PMID 10496258 and PMID 10204850 ). Additional migraine types are suspected and could be proven to be genetic. Migraine understood as several or many disorders could explain the inconsistencies, especially if a single patient has more than one genetic type. However, still other migraine types might be functionally acquired due to hormone organ disease or injury. Three quarters of adult migraine patients are female, although pre-pubertal migraine affects approximately equal numbers of boys and girls. This reveals the strong correlation to hormonal cycling and hormonal-related causes or triggers. Hormonal migraine is a likely consequence of periodically falling hormone levels causing reduction in protein biosynthesis of metabolic components including intestinal tract serotonin.