Which prescription notation indicates cylinder power?

The notation that indicates cylinder power in an optical prescription is the "cyl" value. This value specifically represents the amount of astigmatism correction needed in the lens. Astigmatism occurs when the eye does not focus light evenly on the retina due to an irregular shape of the cornea or lens, and the cylinder power is crucial to addressing this refractive error.

The "sph" value refers to the spherical power in the prescription, which corrects nearsightedness or farsightedness but does not pertain to astigmatism. The "add" value denotes additional power added to the lower part of a multifocal lens, primarily for presbyopia correction, rather than indicating any cylindrical correction. The "axis" value accompanies the cylinder power and specifies the orientation of the astigmatism correction, but it does not represent the strength of the cylinder itself. Thus, the "cyl" value is the essential component for denoting cylinder power in a prescription.