Celluloid and Motion picture

Celluloid noun - Highly flammable substance made from cellulose nitrate and camphor; used in e.g. motion-picture and X-ray film; its use has decreased with the development of nonflammable thermoplastics.

Motion picture and celluloid are semantically related in movie topic. In some cases you can use "Motion picture" instead a noun "Celluloid", when it comes to topics like presentation of action on continuous film.

Motion picture noun - A story told by means of a series of continuously projected pictures and a sound track.

Celluloid and motion picture are semantically related in movie topic. You can use "Celluloid" instead a noun phrase "Motion picture".