Clay Shirky has some interesting discussion of expertise in a larger article on a new wiki-like experiment, Larry Sanger, Citizendium, and the Problem of Expertise:
[E]xperts are social facts — society typically recognizes experts through some process of credentialling, such as the granting of degrees, professional certifications, or institutional engagement. We have a sense of what it means that someone is a doctor, a judge, an architect, or a priest, but these facts are only facts because we agree they are. If I say “I sentence you to 45 days in jail”, nothing happens. If a judge says “I sentence you to 45 days in jail”, in a court of law, dozens of people will make it their business to act on that imperative, from the bailiff to the warden to the prison guards. My words are the same as the judges, but the judge occupies a position of authority that gives his words an effect mine lack, an authority only exists because enough people agree that it does.
In other words, I can tell you that I am an expert in knowledge management, but that doesn't mean much unless this is verified both by my demonstrating expertise as well being validated by the larger context around me: can I provide client references; do I have a definition that meshes with yours; do I participate in KM communities; do I have credentials of some sort... Depending on your own needs and perspective, you will explore some variety of these.
In the comments, Clay clarifies that he is talking about the designation of being an expert, not about expertise or skill.
This leads me to another article that has been sitting in my back pocket, The Expert Mind by Philip E Ross in the August 2006 Scientific American. This is very clearly about expertise (with a focus on chess).
Without a demonstrably immense superiority in skill over the novice, there can be no true experts, only laypeople with imposing credentials.
This articles covers the research into how people develop and demonstrate expertise. At the root is what one researcher calls "effortful study" of the skill in question.
So, an expert is defined by how the world sees her expertise. Clear?