What is in the definition of cluster headache?

Cluster headache. It may occur every day, about the same time, for days, then go away for weeks. It is a “cluster” of attacks. It lasts for hours. It occurs around or behind one eye. It is sharp, it is painful, and it is often associated with the shedding of tears from that eye, or nasal stuffiness on one side, drooping of the eyelid, and sometimes redness of the affected eye.
Cyclical/clusters. They occur in cyclical patterns or clusters. Some describe it as the most painful headache causing night awakenings with pain around the eyes and one side of the head. Frequent bouts of these severe headaches may last weeks to months and sometimes disappear. They are fairly rare and non-life threatening.