Astigmatism

refractive error characterized by the optics of the eye to focus a point object into a sharp focused image on the retina, has_symptom blurred vision.

Astigmatism is defined as the eye creating two images instead of one image on the retina. This happens when the power of the eye isn't the same in all directions, often because the front of the eye (the cornea) is shaped more like the back of a spoon than a perfect sphere. Astigmatism does not affect the health of the eye, and eyes with astigmatism can be perfectly healthy. The result is that objects look blurry, but how much blur is different in different directions, which is why people with astigmatism often have trouble reading some letters but not others.

Astigmatisme B
Astigmatism text blur

It can also cause headaches and eyestrain. In most cases it is easily fixed with glasses, contact lenses, or surgery.