Onion skinning is a 2D computer graphics term for a technique used in creating animated cartoons and editing movies to see several frames at once. This way, the animator or editor can take decisions on how to create or change an image based on the previous image in the sequence.
In traditional cartoon animation, the individual frames of a movie were initially drawn on paper over a light source. The animators (mostly inbetweeners ) would put the 'previous and next drawings exactly beneath the working drawing, so that they could draw the 'inbetween' to give a smooth motion.
In computer software, this effect is achieved by making frames (semi) transparent and projecting them on top of each other. Deluxe Paint was one of the earliest consumer programs to achieve this effect.
See also: CinePaint