A cel is a transparent sheet of plastic foil on which the image sequence of the animated film is drawn, painted or otherwise created. The word cel is derived from celluloid, the material used in the production of the very first animated films.
Celluloid is the common name for a class of compounds created from cellulose nitrate and camphor, with added dyes and other agents. Because cellulose nitrate is highly flammable and prone to rapid degradation, celluloid was soon replaced by the safer cellulose acetate and eventually by the more stable polyester. The term ‘cel’ has become the generic term for all transparent foils used in the production of analogue animated film.