Final answer:
Transparent 'cels' refers to a type of clear plastic material known as celluloid sheets that were used in traditional animation as a medium for creating animated frames.
Step-by-step explanation:
The subject of the question is Transparent 'cels'. Transparent cels, also known as celluloid sheets, are a type of clear plastic material that was commonly used in traditional animation. They were used to create individual frames of animation by hand-painting or drawing the characters or objects on the cels and then layering them over a static background. This technique allowed for the illusion of movement when the cels were photographed or filmed in sequence.