what does cylinder mean for glasses

In the context of glasses, "cylinder" refers to the amount of lens power needed to correct astigmatism. Astigmatism is a condition where part of the cornea has a different curve, causing the eye to be oval or egg-shaped, or more like a North American football, with one curve longer than the other. The cylinder value represents the difference in the greatest and weakest powers of the eye, usually separated by 90 degrees. If nothing appears in the cylinder column, either you have no astigmatism, or your astigmatism is so slight that it is not necessary to correct it with your eyeglass lenses. The cylinder power value may be preceded with a minus sign (for the correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism) . If a cylinder power value is included in your glasses prescription, there will always be a value in the axis column, which indicates the angle (in degrees) between the two meridians of an astigmatic eye.