I've always used term as a collective descriptor encompassing all of the individual chemicals, chemical processes, equipment, voodoo, and magic that makes film photography possible. Sort of like the second definition here.
As in, the chemistry of photography.
PE, whadaya' say?
Yes, I can understand why the word chemistry is used and how it fits into such context. But you will never, ever hear a "proper" chemist speak that way. PE might not count because he was deeply immersed in the photography world and may have been corrupted by you people.