Astigmatism
A condition where the image falling on the retina is not equal in all meridians, causing blurring. It is caused by either the front or back of the eye being non-spherical (i.e. instead of being round, the surfaces may be shaped more like a rugby ball). This prevents the rays from forming a sharp focus. Generally the condition is simply corrected by the use of astigmatic or toric lenses which themselves have different powers over their surface correcting the fault within the eye. Astigmatism can equally affect both Myopic (short sighted) or Hyperopic (long sighted) patients.
One type of chart used to detect astigmatism uses a series of lines arranged in a fan shape - if you have astigmatism, some lines will appear clearer than others.
If you don't have astigmatism (or it is corrected), you can see what it is like by looking at the chart while squinting your eyes. When you do this, the pressure from your eyelids distorts your corneas, causing some temporary astigmatism. When you open your eyes again, your cornea returns to its normal shape.