Note: This was taken directly from VS Battles. Please give them the credit they deserve.
An afterimage is an artistic effect where a character appears to leave faint silhouettes behind them, which are often transparent, like a ghost. These are usually used to emphasize fast movement, and are often used in combat to mislead opponents as to the actual location of the user. It naturally occurs when a character is moving too quickly for the eye to track accurately, causing the brain to incorrectly process these images. At best, the only speed such feats can qualify for in themselves is in the Subsonic range - some people may claim that afterimages mean a character is moving faster than light, but that is both faulty reasoning and an outlier for many characters regardless.