Astigmatism is the term given when the degree of nearsightedness (myopia) or farsightedness (hyperopia) varies as it is measured through different meridians of the eye. In most cases it is due to the front of the eye, the cornea, being out of round; not perfectly spherical.  It is primarily the curvature of the cornea that bends or refracts light.  If a cornea is perfectly spherical, then light is refracted to one precise point of focus.  If that point of focus falls short of the retina, then the eyeball is too long.  We call this condition myopia.  If the point of focus falls behind the retina, then the eyeball is too short.  We call this condition hyperopia.  With astigmatism, there is no precise point of focus.  Since the cornea is out of round, light is bent or refracted to an infinite number of different points of focus.  The result is blurred, distorted vision.  Uncorrected, astigmatism causes eye strain and can be a contributing factor to headache. Most people who wear an optical correction, be it eyewear or contact lenses, have some degree of astigmatism.