It seems that a film is anything that may be put in a projector that will modulate the emerging beam of light.
“You can write on a wall with a fish heart, it’s because of the phosphorous. They eat it. There are shacks like that down along the river. I am writing this to be as wrong as possible to you. Replace the door when you leave, it says. Now you tell me how wrong that is, how long it glows. Tell me.”
Anne Carson | “Short Talk on Shelter”