Coma is a shortcoming in a camera lens where light rays, bumping into the objective far from it's optical axis do not come to focus within the identical image plane. This results in star images near the outer edge of the viewing field seeming to have comet-style tails scattering radially away from it's optical axis (called negative coma) or toward it's axis (called positive coma).
Coma is a natural occurring property of telescopes which use a parabolic mirror. Light originating from a source (like a star) within the middle of a field is flawlessly focused at it's focal point within the mirror (different than a spherical mirror, which has light coming from the outer portion of the mirror focusing nearer to the mirror over illumination from it's center, called spherical aberration). Although, when the source of light is not-centered (off-axis), diverse portions from the mirror don't reproduce the light on to the identical point. Resulting in a light point that is un-centered in the field and seemingly shaped like a wedge. The more off-axis, the more the effect worsens. This makes stars seemingly having a cometary coma, thus the name. Various schemes to diminish spherical aberration without instituting coma include Maksutov. Ritchey-Chrétien, Schmidt, and
other optical schemes.
Coma within a lens system or a single lens can be reduced (and in a few situations eliminated) by selecting the lens curvature surfaces to go with the application. Lenses in where both coma and spherical aberration are reduced within one wavelength are called aplanatic or best form lenses.