A tension headache (renamed a tension-type headache by the International Headache Society in 1988) is the most common type of primary headache. The pain can radiate from the neck, back, eyes, or other muscle groups in the body. Tension-type headaches account for nearly 90% of all headaches. Approximately 3% of the population has chronic tension-type headaches.
( more at : http://en.wikipedia.org/wiki/Tension_headache )