Original means what you think it means. Just as I don't have to explain what is an original painting.
If you want a specific, technical definition it's a photograph made directly from the original negative very soon after the image was shot. The exception to the 'directly from the original negative' is with a wholly unique creation, such as some artistically and meaningfully unique artistic collage or composite image-- and whether or not it is truly unique/original is judged and debated instance by instance. The latter can always be debated as it's an aesthetic/philosophical question as much as a technical one.
The below vintage 1890 cabinet card is a rephotographing of cutout photos, but the design is so creative and unique that I would call it original. If someone doesn't want to call it original, but just vintage that is fine. Doesn't effect the value.
