In lens prescription terminology, what does 'axis' refer to?

In lens prescription terminology, 'axis' specifically relates to the orientation of astigmatism correction within a prescription. It is defined as the meridian where there is no cylinder power, which indicates the position of the lens that will provide the necessary correction for the irregular curvature of the cornea or lens. The axis is measured in degrees from 1 to 180, describing the angle at which the cylindrical lens should be placed to properly align with the patient's visual needs.

Understanding this concept is vital for accurately prescribing corrective lenses. The other concepts—including lens curvature, thickness, and focal point—are relevant to the overall design and function of the lens but do not pertain to the definition of 'axis.' Thus, recognizing 'axis' as the meridian of no cylinder power highlights its essential role in astigmatism correction within ocular prescriptions.