It's all relative.  Your an expert when you can do one specific thing better than 99% of those who attempt it.  As mentioned, a NASCAR or Indy driver can out drive 99% of drivers in the world.  Tiger Woods is the top at his game.  It doesn't mean you "know it all", far from it but it does mean that your one of the best at what you do. 

Of course this is my definition of an expert, yours might be different.  That being said, I'm an intermediate pilot at best.