Cluster headache: A distinctive episodic syndrome of headaches. The most common cluster headache pattern, acute cluster headache, is characterized by one to three short attacks of pain each day around the eyes, clustered over a stretch of 1 to 2 months, and followed by a pain-free period that averages 1 year. The other main pattern of cluster headache, chronic or episodic cluster headache, is characterized by the absence of sustained periods of remission, with pain occurring out of the blue or emerging several years after an episodic pattern. Cluster headache is different and distinct from migraine, although the underlying mechanisms are similar. For example, propranolol is effective in treating migraine but not in treating cluster headache, whereas lithium is beneficial for cluster headache but not migraine. Also known as ciliary neuralgia, erythroprosopalgia, histamine cephalgia, migrainous neuralgia, Raeder syndrome, sphenopalatine neuralgia, and vidian neuralgia.