In optics, the surface connecting similar points on adjacent waves of light among an essentially infinite number of waves traveling together from a light source. The simplest form of a wavefront is called a plane wave, where all light rays are parallel to each other. Distortions of visual wavefronts caused by irregularities of the optical system of the eye are called higher-order aberrations (HOAs). These wavefront distortions can be measured at the plane of the pupil by instruments called aberrometers, which document HOAs in the form of a topographical map. These maps can then be used to correct HOAs with customized, wavefront-guided LASIK.