One diopter of astigmatism refers to the measurement of the difference in optical power between two principal axes of a lens. This measurement indicates how much correction is needed to address the distortion caused by astigmatism. The concept can be visualized by slicing a lens along its axes to create circularly symmetric lenses, with the power difference reflecting the astigmatism level. Additionally, prescriptions may denote astigmatism using a cylinder measurement, such as 0.4Cyl, along with an angle. Understanding these terms is crucial for effective vision correction.