Aberrations characterise various defects of beam passage through an optical system. They can be described using criteria of ray optics or concepts of wave optics. In the first case departure from homocentricity is expressed through geometric aberrations and shapes of ray scattering in point images. In the second case the focus is on evaluating deformation of a spherical light wave passing through an optical system, which introduces the notion of wave aberrations. Both ways of describing are interrelated, they describe the same state and differ only in form of description.

Aberrations fall into two classes: monochromatic (characteristic of monochromatic beams) and chromatic.

Monochromatic aberrations are inherent of all real optical systems. Their appear because refractive surfaces are not able to gather in one point any wide beams falling at large angles. Such aberrations cause an image of a point to blur (form a scattered pattern instead of a point) which negatively effects image definition and breaks similarity between an object and its image. Such aberrations include spherical aberration, coma, astigmatism, image field (surface) curvature, distortion.

Chromatic aberrations are caused by the dispersion of optical media forming the optical system, i.e. dependence of the refractive index of optical materials making up the elements of the optical system on the length of the transmitted light wave. They can manifest themselves in unusual image colouring or "fringes" of colour along the boundaries of the image. Such aberrations include chromatic aberration of position (coordinate chromatism), also called "longitudinal chromatism", and lateral chromatic aberration.