Who is an athlete? What is a sport?

When the Associated Press recently named NASCAR champion Jimmie Johnson its Male Athlete of the Year, it brought up a couple of interesting questions. What is an athlete? And, what qualifies as a sport? It's hard for me to consider a guy who sits down on the job for the title of top athlete. There is no doubt that Johnson has great skills as a race car driver, including tremendous reflexes and great stamina. But I really don’t think he can be considered alongside people like Roger Federer, Usain Bolt, Peyton Manning, Lance Armstrong or Lebron James. That brings us to the question of what is a sport, and what is not. In my definition – and this is just my opinion – a sport involves defense. In other words, there has to be someone trying to stop you from doing what you want to do for an athletic event to be a "sport." To me, golf is an athletic pursuit, as are track and field events, gymnastics and ice skating. They all require athleticism, but they're not sports, at least under my definition. What are your thoughts?