The other day I was asked a very good question - "what makes someone an expert?"

Considering web-based media is moving into newer fields and industries, it's hard to define expertise along the lines of professional certifications and documented qualifications.  An expert accountant will have his CPA.  An expert manager will have her MBA.  A doctor has an M.D.  A professor has a Ph.D.  These are certifications of expertise issued by recognized authorities that state quite clearly: this person is an expert.

There are newer fields, though, that don't have these kinds of certifications.  How do you become an expert in developing applications for Facebook?  What makes you an expert blogger?  Who defines relative levels of expertise for software UI designers?  These are fields where expertise is vital to creating a beneficial working relationship.  What exactly constitutes "expertise" in such an ambiguously defined field?

Think back to when you were a child.  Among children, an "expert" is anyone who knows more about a topic than you.  In most cases for children, it's either a parent or a teacher.  In some cases, though, it's merely an older kid on the playground.  When you were 7, did someone need to be certified to teach their peers the rules to basketball?  Did you check for documentation before asking how to play tether-ball?

When it comes to a new field like blogging, we're all essentially children.  In that case, an expert is anyone who knows more about the art than you.  They'll still need to back up this statement of expertise with a strong portfolio, but experience is the name of the game here.

I've been blogging now for seven years.  I've worked with 6 different platforms, two of which I built myself.  I've managed over 20 different sites for my own purposes and built roughly 50 for other people.  I continue to develop solutions for current platforms and maintain a strict update schedule of 3 posts per week.

Yet I am still reluctant to call myself an "expert."  I know only what I know, and there are people in the market who know more than me.  Still, there are just as many who are learning the ropes and can benefit from my experience and relative expertise.

What constitutes expertise in your field?  If there aren't formal certifications, who in the market would consider you an expert?  Who do you consider an expert in comparison to yourself?