When you look at your eyeglass prescription, you will see numbers next to OD and OS and also listed under Sphere, Cylinder, and Axis. OD mean right eye and OS means left eye. Sphere determines if you are nearsighted or farsighted. Cylinder measures the degree of astigmatism that you have. This number can also be a negative or a positive. Axis is a number between 0 and 180 degrees. It determines the orientation of the astigmatism.
Finally, the ADD power represents the power that needs to be added to the distance prescription to give the patient better vision at a close range. This commonly begins to develop as we reach our 40s.