Why are movies sometimes referred to as a "celluloid" ?

Question

Why are movies sometimes referred to as a "celluloid" ?

Answer 1

Celluloid is the substance out of which early movie film was made

Answer 2

Celluloid is first thermoplastic that the the motion picture photographic film was made out of.  It later evolved to a more flexible and adaptable type of film by the turn of the 20th century. This ability to produce photographic images on a flexible material (as opposed to a glass or metal plate) was a crucial step toward the advent of motion pictures.