“Expertise is said to be acquired through study, education, or experience. However, expertise is often conferred through perception rather than through objective means.
A house painter, for example, is likely to acquire expertise from experience and training, while a nuclear physicist gains the expert appellation by virtue of advanced degrees.
We are looking for someone who can make a decision or give a qualified opinion about how to act relative to some set of circumstances or data. We are looking for someone who knows the facts-someone who has knowledge that we don’t. In the end, it may be that you need not define an expert to identify one. Perhaps we just know, or intuit, when we find an expert and need not worry about the definition.”
Source: Everybody’s an Expert: Finding Business Experts Online, By Roberta Brody & Marydee Ojala; Online, May 1 2008
Someone affiliated with a reputable organization, university, etc.
Someone who has authored works that have been characterized or identified as authoritative in the field in question, by multiple reputable sources.
Someone that has been characterized or identified as an authority in his or her field, by multiple reputable sources.
Someone who by virtue of their position (in a government agency, for instance) could be considered to be an authority.
No matter where you find a source is listed, check them out. Do your due diligence so that you are not embarrassed later. Run a Google search and a news clip search to look for any controversies, and also see what expert has said about your subject in the past to look for glaring inaccuracies or bias exhibited.