Animation stills are a type of still image used in the animation industry. They are typically used as reference material for animators, and are created by taking a photograph of a single frame of an animated sequence. The term 'CEL' is an acronym for celluloid, which is a type of transparent sheeting used to create animation cels. Cels are painted on one side and placed over a background image, creating the illusion of movement when the CEL is moved.