It is not at all easy to say what an expert is. This is actually studied in epistemology under the general heading expert knowledge. On one account, put very roughly, an expert is a person who is (or, perhaps, is widely recognized to be) a reliable (see reliabilism) source of knowledge by testimony for other people.
In the field of medicine, a movement called "evidence-based medicine" ranks quantitative empirical analysis, such as double blind randomized controlled trials, above the judgements of experts, ie. medical professionals.
Experts can be contrasted with crankss, who think they know what they are talking about, but don't, and with academic elitism, a syndrome of those who think that ONLY they know what they are talking about, and no one else does.