A prism is a shape with flat, transparent surfaces that bends light. Some people suffer from double vision caused by their eyes not focusing on the same location. Adding a prism shape to eyeglass lenses bends light travelling into the eye in such a way that it aligns what each eye is seeing, eliminating the double image interpreted by the brain.

If you need prism correction, you will have a separate set of numbers on your prescription that indicate the vertical and/or horizontal angle needed to correct double vision.