A headache is a pain or discomfort in the head, often described as a throbbing or pulsating sensation. It can be caused by various factors such as stress, tension, lack of sleep, dehydration, eye strain, sinusitis, migraines, and other medical conditions. Headaches are typically classified into two types: primary headaches (such as tension or migraine) which have no identifiable cause, and secondary headaches (such as those caused by high blood pressure or injury).