Astigmatism is an assymetry - lopsidedness - of the cornea (or in some cases the lens of the eye) which causes light to not focus properly on the back of the eye (retina), thereby messing up your vision in such a way that you need glasses or contact lenses.
You may be short/nearsighted (myopic) or long/farsighted (hyperopic) and also have astigmatism. Like myopia and hyperopia, astigmatism is measured in dioptres, but it also has an axis (location). In a glasses or contact lens prescription, the second number (if any) indicates the amount of astigmatism and the third number indicates the axis.
As a type of refractive error, astigmatism is one of the things laser eye surgery seeks to correct. Large amounts (say more than 1.5 dioptres) of astigmatism are a serious challenge for laser eye surgery however and are associated with a lower rate of success and higher enhancement rates.
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