What does 'Cylinder' refer to in optical terms?

In optical terms, 'Cylinder' refers specifically to the power needed to correct astigmatism. Astigmatism is a common refractive error caused by an irregular shape of the cornea or lens, which leads to blurred or distorted vision. The cylinder measurement indicates the degree of astigmatism and is part of a lens prescription, detailing how much extra power is required in one meridian as opposed to the other to achieve clear vision.

Understanding this concept is crucial for properly fitting corrective lenses, as the cylinder value helps determine the appropriate type of lens that will compensate for the asymmetrical curvature of the eye. It is important to distinguish that this term does not relate to distance or reading correction specifically, nor does it describe the physical curvature of contact lenses.