Celluloid is a type of plastic that’s transparent and flammable. Until recently, most movies were filmed on celluloid.
Celluloid is a name for film used in shooting movies. Because of its use in making films, this term came to stand for movies in general. Its days in the projection room are now pretty much finished — due to the rise of digital filming — but the figurative meaning lives on, wherever stars aspire. For example, you could say a TV actor was making the move from the small screen to celluloid. Besides movies, celluloid is used in shooting x-rays.