[Nadya]

You're right that enthusiast doesn't have any dictionary definition with the implication I mentioned. So for lack of a dictionary term, allow me an analogy that also explains why it isn't a "no true scotsman" argument.

Compare game genres to musical instruments. Guitarist == FPS gamer and Pianist == RPG gamer

Both guitarists and pianists are musicians, just like RPG players and FPS players are gamers. Feel free to expand this analogy into as many genres and musical instruments as you like.

Now take someone who isn't able to play the correct notes, cannot read sheet music/tablature/any form of music notation, has no knowledge of music theory, hasn't heard of The Beatles (or whatever cultural/genre relevant artist would be the equivalent to The Beatles), but still practices for hours every day on their 8-note plastic recorder they got in 4th grade [0]. They call themselves a musician, after all they spend so much time with their hobby!

Nobody but that person and maybe their mother - being nice - would call them a musician. Is there a hard, defined line for when they'll "become a musician"? No. There is a grey line of necessary skill/knowledge that one needs to have for others to consider them a musician. Every amateur hobby has that grey line that needs to be crossed before you'll be taken seriously.

ps. The dictionary definition of "musician" only mentions "plays an instrument" and doesn't indicate any level of skill. This is the difference between "dictionary definition" and "actual usage of a word". By the dictionary's definition our untalented individual is a musician, even if nobody else would consider them one.

[0] http://i.imgur.com/UXHmy6c.png

[WorldMaker]

That analogy doesn't work for me either. History is littered with amazingly beloved musicians that played entirely by ear, with little to no literacy in the larger world of music, that basically reinvented everything they knew of music from first principles with never studying the previous culture or proper music literacy or music theory.

I think the dictionary definition is accurate/adequate enough and you may have an implicit "good" or maybe "professional" somewhere in your usage of musician, that I don't. One is a value judgment (history is also littered with "terrible" musicians that were still musicians, or contributed to the craft) and the other an economic judgment, neither of which I see as necessary to describing who is or is not a musician.

[Nadya]

We'll have to agree to disagree here, because I don't find dictionary definitions all that useful for communication and most people don't operate under strict dictionary definitions.

By definition a pilot doesn't need to successfully fly - only operate the controls. So a drone pilot could crash every drone and still be a "pilot" by definition. Nobody would recognize them as a drone pilot because there is an implied "successfully" that isn't found in the definition.

Yes - there is an implicit "at least to some level of success" in my definition. "Professionals" meet that criteria by being good enough to be paid for what they do. Amateurs come at many different levels but I don't call myself a photographer just because I've taken a few (hundred) photos. This is where we disagree - because you would consider me a photographer for having taken any photos.

[WorldMaker]

Here's where your analogy particularly breaks down: "a gamer" is neither a professional nor an amateur mark. Fandom does not, and perhaps cannot, have any sort of success bar. There is a notion of a "professional gamer" in the eSports world, and it's possible to extrapolate thereby to a notion of an "amateur gamer" that competes in eSports. But that belies a confusion between "[sports] gamer" and "[fandom/enthusiast] gamer". Within the context of fandom/enthusiasm, what would "professional" mean? "Amateur"?

Fandom/enthusiasm don't really have success bars. It's something you are either enthusiastic about or you aren't. You can be a fan of something and never be successful at it, however you define success. A baseball fan doesn't have to be good at actually playing baseball nor devoted to a deep knowledge of the sport to be a fan of their favorite team.