What is astigmatism?  

Astigmatism is a refraction defect, caused by a deformation of the cornea.

The cornea, the transparent anterior surface of the eye, is almost spherical in a normal eye, whereas in a person suffering from astigmatism, is a bit deformed by the flattening of some of its axis. This causes a defocused image, partially compensated by the focusing of the lens causing ocular tiredness, in long-distance as well as in short-distance.

Even though astigmatism can come up as an isolated defect, in most cases it’s combined with myopia (short-sightedness) or hypermetropia (Hyperopia or long-sightedness). The amount of corneal astigmatism does not vary in a lifetime, except for rare corneal diseases, but the ability to compensate decreases with time.