celluloid [from cellulose ], transparent, colorless synthetic plastic made by treating cellulose nitrate with camphor and alcohol. Celluloid was the first important synthetic plastic and was widely used as a substitute for more expensive substances, such as ivory, amber, horn, and tortoiseshell. It is highly flammable and has been largely superseded by newer plastics with more desirable properties. It has been used for combs, brush handles, billiard balls, knife handles, buttons, and other useful objects.