In optics (especially telescopes), the coma in an optical system refers to monochromatic aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted.
Coma is an inherent property of telescopes using parabolic mirrors. Light from a point source (such as a star) in the center of the field will all come together at the focus of the mirror (unlike a spherical mirror, where light from the outer part of the mirror focuses closer to the mirror than light from the center--spherical aberration). However, when the light source is off-center (off-axis), the different parts of the mirror do not reflect the light to the same point. This results in a point of light that is not in the center of the field looking wedge-shaped. The further off-axis, the worse this effect is. This causes stars to appear to have a cometary coma, hence the name. Schemes to reduce spherical aberration without introducing coma include Schmidt, Maksutov and Ritchey-Chrétien optical systems.
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