Hope this helps...or maybe futher add to the confusion
Authentic and Original...generally, both terms are used interchangebly...as far as sellers are concerned. However, this does not mean that everything is smooth sailing fom this point...it can get a bit complicated
Best is to do as much research (via the net, books, mags, etc) when mind is set to buy a watch.
Example: A Rolex watch, a particular model may be released in several variations...like different bracelet, bazel and dial...yet there are cases which the watch ends up being a `rojak'. Owner decides to change/replace things...at the end, the watch may have different Rolex parts which may be `original'...yet the `authenticity' of the particular model is compromised.
Issues that may also be a point of concern will be things like aftermarket, custom fitted and refurbished.
Doing a PVD (Physical Vapor Deposition) to an original watch will also compromise its authenticity.
Note: comments from a newbie to this forum.