From Discworld & Terry Pratchett Wiki
Octo-cellulose is the Discworld equivalent of celluloid, invented by Thomas Silverfish of the Guild of Alchemists. This stuff plays a pivotal role in the events of Moving Pictures and in the development of the Moving Pictures industry. It is very flammable, and burns very quickly, and this is something that is used to Victor Tugelbend's advantage when it comes time to stop the showing of Blown Away. This flammability also causes the denizens of the Dungeon Dimensions who break into reality during the showing to fear fire- even magical, illusory fire.
It may or not be related to a later development by the Alchemists- that of synthesizing ivory into billiard balls- when a Roundworld reference is taken into account (see the annotation). During Men at Arms, the Alchemists are found by Cuddy and Detritus playing Billiards. The billiard balls- which they have created themselves, without the use of ivory- are notable for their volatility: during the game they accelerate frighteningly, explode into flame, and whistle around the room.
In the late 1860s, the then-phenomenal prize of $10,000 was offered for anyone who could produce a substitute for ivory, specifying that it had to conform exactly to the propensities of a billiard ball. An American named John Wesley Hyatt created arguably the first industrial plastic. Whilst experimenting with Parkesine, a material first created by Alexander Parkes (from Birmingham, England), he came up with a commercially viable way of producing Celluloid which he patented in 1869. He coated composite billiard balls with it and entered for the $10,000. This added considerably to the excitement of playing billiards - when two balls connected, there was a sharp crack! just like the report of a pistol. If that isn't a Pratchettism...