<unk> s fall into two classes:
Monochromatic<unk> s (Gr. monos, one) produced without dispersion.  These include the<unk> s at reflecting surfaces of any colored light, and at refracting surfaces of monochromatic light of single wavelength. These include:
Piston
Tilt
Defocus
Spherical
Coma
Astigmatism
Curvature of field
Image distortion
Chromatic<unk> s (Gr. croma, color), where a system disperses the various wavelengths of light
Axial, or longitudinal, chromatic<unk> 
Lateral, or transverse, chromatic<unk> 
Piston
Tilt
Defocus
Spherical
Coma
Astigmatism
Curvature of field
Image distortion
Axial, or longitudinal, chromatic<unk> 
Lateral, or transverse, chromatic<unk> 
Piston and tilt are not actually true<unk> s, as they do not represent or model curvature in the wavefront. If an otherwise perfect wavefront is "aberrated" by piston and tilt, it will still form a perfect,<unk> -free image, only shifted to a different position. Defocus is the lowest order true<unk> .
Optical aberration
Vascular anomalies
Electrical injury
Refractive error
Neoplasia
Congenital abnormality
Vascular malformation
Lymphatic malformation