It seems that a film is anything that may be put in a projector that will modulate the emerging beam of light.
“If you catch a dragonfly and bend the end of its body up, it will eat itself until it dies. When people have had their mouths numbed for dentistry, they must be warned not to chew their tongues. I think if we assume that pain is simply an evil we’re oversimplifying things.”
Gene Wolfe