What makes someone an expert at poker?

[Mr. Curious]

The word "expert" is bandied about quite often and I am wondering what makes people consider others (or themselves) to be an "expert"? Some friends and I had a discussion where there were four levels:

Beginner
Advanced
Expert
Master

But we didn't really clarify the line between the four categories. So what do you think makes an expert and how big is the difference between each of the levels?

Note: This question occurred to me after reading GSIH again and Ed's comment that some people will think they are experts after having a couple of winning months.