APA Dictionary of Psychology

an extremely severe unilateral headache, typically limited to the area around one eye, that lasts between 15 minutes and 3 hours and occurs in bouts, or “clusters,” every day (sometimes twice or more a day or every other day) for a period of up to 3 months, followed by a headache-free period of months or years. The exact cause is not known but may include vascular dilation, trigeminal nerve activity, or histamine release. Also called histamine headache.