CNN looked at the lifestyle of "MVP"--the world's top ranked StarCraft II player. For this fierely competitive Korean man, StarCraft isn't just a game, but a challenging event that demands rigorous training. CNN's John D. Sutter asks:
How do you draw the line between obsessive, unhealthy behavior and a drive to be the world's best? Should pro gamers (or chess players, for that matter) be put in the same class as track-and-field stars and gymnasts, who also go to extreme lengths to get in shape for competition? Many pro gamers in South Korea, for example, are required to run, swim and lift weights as part of their training.
Link -via Glenn Reynolds
To me it means someone involved in a sport, a sport being an activity that uses physical abilities.
But if we take "athlete" to mean what it etymologically should, i.e. a contestant for a prize, then obviously pro gamers qualify.
and i expect that my definition of sports / athletes may also be valid for them:
Sport is a leasure time acitvity with a doping problem in the top competitions...