Professional

"Pro" is not the opposite of "fan".

In fandom the word pro or "professional" is used in multiple ways, but the basic distinction between "pro" and "non-pro" is receipt of more than de minimus payment for work.

A specific piece of work is pro if the person who produced it was paid more than a token amount for it. So the common fannish payment for a contribution to a fanzine of a beer or a copy of the zine is token and does not make the work pro.

The best way to understand what is token and what isn't is to ask if the payment would be a significant motivation for someone to consider doing the work. Payment in cash is usually non-token. Cash prizes in a contest where only a small fraction of the entrants can expect to win a prize is not normally considered to make the contest entry professional.

Token payment, even in cash, does not make something professional. A work is professional when the payment involved creates a non-trivial economic justification for doing the work. (Note that we're talking motivation here, so that what is a good salary for a job for J. Random Poorfan, is an entirely token payment for Bill Gates.)

The semi-pro magazines are a particularly complex case because they normally do pay cash, but the amount is often very small. As of 2012, when the professional magazines are paying 3-5 cents/word for fiction, payments of around 1 cent/word or higher are generally considered to be professional. (Lower rates may also be, but it's not as cut-and-dried.) There is a continuum between semi-professional magazines and fanzines which publish fiction.

Someone who is known for their professional work will often be called a pro (which does not preclude them from also being fans or engaging in fannish activities, even in their field of professional endeavor if they are not paid for it).