An operation that can improve or correct the ability of the eye to focus by permanently changing the shape of the cornea (the clear outer layer of the eye). Refractive surgery may eliminate the need to wear glasses or contact lenses or reduce the strength of the correction. Various types of refractive surgery can be used to correct nearsightedness (difficulty seeing distant objects), farsightedness (difficulty seeing close objects), and astigmatism (tilted, distorted vision due to an irregularly shaped cornea).