Coma
An aberration that renders off-axis points of light as asymmetric comet-shaped smears rather than dots.
In depth
Coma grows with distance from the optical axis and with aperture, so it is a corner defect of fast lenses used wide open. It matters most in astrophotography, where every star in the corners becomes a small wing-shaped blob, and it is the single specification night-sky photographers scrutinise before buying a wide-angle lens.
Examples in practice
Corner stars in a Milky Way frame at f/1.4 appear as tiny seagull shapes; by f/2.8 they are round again.