A gamer who uses competitive gaming (tournaments, leagues, etc.) as their sole source of income. This does not refer to someone who games all the time and attains some level of skill; being a pro gamer means you use it as your profession, and that you actually do earn money. In the USA, there are few pro gamers, and those that do exist usually make all of their money from prizes in tournaments and leagues, with some income coming from sponsorships. In Korea, however, there are actually leagues with paid salaries, closer in format to baseball.
"Wow I just beat a good player, I'm such a fucking pro gamer."
"No you aren't, you don't even make any money you fag."