What does "minus" cylinder (cyl) power correct?

The term "minus" cylinder power refers specifically to the correction of astigmatism. Astigmatism is a refractive error caused by an irregular shape of the cornea or lens, leading to distorted or blurred vision at various distances. The minus cylinder notation is used to describe the lens shape needed to counteract the uneven curvature of the eye, allowing for focused light on the retina and improving the clarity of vision.

In this context, the other options do not align with the function of minus cylinder power. Myopia (nearsightedness) typically requires a minus spherical power, while hyperopia (farsightedness) involves plus spherical power. Normal vision does not require corrective lenses. Therefore, minus cylinder power is specifically designated for the correction of astigmatism, making it the correct selection for this question.