A headache (medically known as cephalalgia) is a condition of mild to severe pain in the head; sometimes upper back or neck pain may also be interpreted as a headache.

Headaches have a wide variety of causes, ranging from eyestrain to inflammation of the sinus cavities to life-threatening conditions such as encephalitis, brain cancer, meningitis, and cerebral aneurysms. When the headache occurs in conjunction with a head injury the cause is usually quite evident; however, many causes of headaches are more elusive. The most common type of headache is a tension headache. 

There are four types of headache:  vascular, muscle contraction (tension), traction, and inflammatory.  The most common type of vascular headache is migraine. Migraine headaches are usually characterized by severe pain on one or both sides of the head, an upset stomach, and, at times, disturbed vision.   Women are more likely than men to have migraine headaches.    After migraine, the most common type of vascular headache is the toxic headache produced by fever.  Other kinds of vascular headaches include "cluster” headaches, which cause repeated episodes of intense pain, and headaches resulting from high blood pressure.  Muscle contraction headaches appear to involve the tightening or tensing of facial and neck muscles.  Traction and inflammatory headaches are symptoms of other disorders, ranging from stroke to sinus infection.  Like other types of pain, headaches can serve as warning signals of more serious disorders. This is particularly true for headaches caused by inflammation, including those related to meningitis as well as those resulting from diseases of the sinuses, spine, neck, ears, and teeth.

Types of headache

• Tension headache: are the most common type of headaches. The pain can radiate from the neck, back, eyes, or other muscle groups in the body. Nearly everyone will have at least one tension headache in their lifetime.
• Migraine: Migraine headaches are vascular in origin, severe, occur on one or both sides of the head, and are often associated with various symptoms such as nausea, vomiting, sensitivity to light, or disturbed vision. Certain types of migraines are preceded by neurological symptoms, known as an aura.
• Cluster headache: are rare headaches that occur in groups or clusters. Cluster headache sufferers typically experience very severe headaches of a piercing quality near one eye or temple that last for between 15 minutes and three hours. Cluster headaches are frequently associated with drooping eyelids, red, watery eyes, and nasal congestion on the affected side of the face. The headaches are unilateral and occasionally change sides. The neck is often stiff or tender in association with cluster headaches, and jaw and teeth pain is sometimes reported.
• Rebound headaches: occur when medication is taken too frequently to relieve headache pain. Rebound headaches frequently occur daily and can be very painful. A diagnosis of rebound headaches can be easy or difficult, as the cause is very easy to identify but very difficult to diagnose. Overuse of painkillers can be confirmed simply by asking the patient if his or her headaches assumed a new pattern or became more severe after taking painkillers excessively (generally classified as more than 3 times per week). However, the only way to make a certain diagnosis of rebound headache is to withdraw the patient from medication for anywhere up to 6 months. It should be noted that withdrawal from medication will actually intensify the headaches for the first few weeks. After this period, the headaches will gradually recede.  Following treatment, many patients revert to their prior headache pattern. A physician should be consulted before re-use of medications.
• Ictal headache: are headaches associated with seizure activity. They may occur either before (pre-ictal), after (post-ictal), or most rarely during a seizure. Many cases of ictal headache may be misdiagnosed as migraine with aura, or even cluster headache. However, whereas these conditions usually involve just one side of the head (are unilateral), an ictal headache may be centrally situated or cover the entirety of the head.
• Ice cream headache: Brain Freeze or ice cream headache are term used to describe a form of cranial pain or headache which human beings are known to sometimes experience after consuming cold beverages or foods such as ice cream, often as a result of consuming at a high rate of speed.  In rare instances, neuro-cranial blood vessels can constrict so completely that a temporary blackout can occur.
• Thunderclap headache: is a sudden and severe headache, diagnosed via a process of exclusion with accompanying negative CT and lumbar puncture results.  Thunderclap headaches can be indicative of life threatening medical problems.