I think the definition and threshold for expertise is relative and varies depending on specifically what types and sets of coins you are talking about. I also think this is true in sciences, and for knowledge in general. So in my opinion, an expert has considerably more knowledge than the average person familiar with the subject.
For example, lots of collectors are familiar with
Mercury dimes. They're one of the most popular and well known types around. So the average expanse of knowledge on the subject is such that most coin collectors would know following in some sort of order of common familiarity:
- These dimes were minted during World War II
- They were minted 1916-45
- Being able to grade them on a scale of 4-60
- 1916D is the key date
- 1921 P/D are semi-keys
-
Adolph Weinman designed them
- 1942/41 P/D are key date varieties
I would venture a guess that most here probably know all of that. That's one level of knowledge on
Mercury dimes. Here's perhaps the next:
- Knowing off the top of your head the dimensions and weight
- Rough (or perhaps fairly exact) mintages for key dates
- Knowing common errors and varieties
- Being aware of FSB designations
- Being able to differentiate between 63-68 grades
Even that's not really enough knowledge to be considered an "expert" on
Mercury dimes, and likely doesn't even bump you into say the top 25% when it comes to knowledge of the type just on this forum. I'm certainly not an "expert" on
Mercury dimes, so I couldn't even say how you would then define the next strata of knowledge, but there are certainly members here who could do so easily. Now contrast that to something like Medieval English or French coins where just being able to read legends, identify rulers, mints, and using varieties to pin down a more specific date range probably has a much greater chance of putting you in the top 10% of knowledge on the subject on the forum, and yet that is comparatively basic knowledge when it comes to
Mercury dimes.
The more specialized the subject, the smaller the knowledge pool, so the threshold for being even a "resident expert" drops considerably. I'm not sure that I'm effectively articulating my thoughts and opinion on this, but I suspect that one should get the gist of what I'm trying to convey. In some ways, this is similar to what DrDarryl just said.
In my mind the same is true in sciences. It's a lot harder to be the expert on something like chemistry or physics in general as it is to become an expert on say cleavage techniques of superconducting materials under ultra-high vacuum where the pool of knowledge is considerably smaller. Having been in academia as well as industry in sciences and engineering, I'd be careful to put too much emphasis on a requirement of publication to define expertise. Lots of names are appended to papers that have had disproportionately low contributions to their peers. Conversely, it is often the case that while the principal investigator may have their name at the top of the paper, it may have been graduate students that did the overwhelming majority if not all the work only to receive a fraction of the acknowledgement and notoriety. Additionally, lots of work is published just for the sake of publication with reciprocity between universities, donors, and the grants that are given to departments on a regular basis. I don't want to digress too much, but the point being that taking publication and its volume at face value doesn't always give a clear picture as to the merit of the work. Sometimes one landmark discovery is worth a thousand other publications! I say all of this and use the analogy of scientific research as it is something I am far more familiar with than numismatic research and I think the same ideas can be applied to numismatic expertise as well.