The eye has a cornea — a clear, curved layer that protects it. If the cornea's shape is irregular, astigmatism results. People who have astigmatism see images as distorted, so they need to wear glasses or contact lenses.
Sometimes people mistakenly say, "One of my eyes has a stigmatism." But the a in astigmatism is important to its meaning. The prefix a- means "without." Stigma is Greek for "point," so an eye that is "without point" is one that doesn't allow light rays to meet at a single point or focus. Since astigmatism is a condition, you don't need the article an before it: "My eye has astigmatism," not "My eye has an astigmatism."
