headache - A headache (medically known as cephalalgia, sometimes spelled as cephalgia) is a condition of pain in the head; sometimes neck or upper back pain may also be interpreted as a headache. It ranks amongst the most common local pain complaints. Headaches have a wide variety of causes, ranging from eye strain, sinusitis and tension to life-threatening conditions such as encephalitis, meningitis, cerebral aneurysms and brain tumors (although up to 99% of headaches have a benign cause and do not warrant further treatment other than a painkiller).