Keeping this in mind, I would certainly include Daguerreotype, Ambrotype,
tintype, albumen prints, polaroid transfer, photo-silkscreen, emulsion
transfer, gum, cyanotype, kallitype (and VDB), platinotype, paladotype,
flourotype, uranotype, gum prints, casein prints, gumoil, etc. etc.
You get the picture. If you want a longer list, see Luis' encyclopedia.
I do not believe there exists a formal definition other than what those
of us practicing "the art" believe it to be and I'll bet the responses
demonstrate that there is no general agreement about this.
Some time back there was a very interesting discussion about what we
should call ourselves. That was never settled, but it was fun.
Keep on printing,
Bob Schramm