Define "astigmatism."

Astigmatism is a refractive error that arises due to an irregular curvature of the cornea or lens in the eye. This irregular shape prevents light from focusing properly on the retina, leading to distorted or blurred vision at various distances. The condition can affect both near and distant vision, making it essential for individuals with astigmatism to receive the appropriate corrective lenses.

The other options, while related to vision or eye conditions, do not accurately define astigmatism. For example, the second option refers to a specific vision issue that typically occurs at night, which is not inherently related to the irregular shape of the cornea or lens. The third option describes presbyopia, a common age-related condition that affects focus but is distinctly different from astigmatism. Lastly, the fourth option suggests that astigmatism is a disease, whereas it is actually a refractive error rather than a pathological condition. The emphasis on the spherical or cylindrical shape of the eye with respect to astigmatism is what differentiates it from other vision-related issues.