Expert: a person with a particular skill, special knowledge, or a person who knows a lot about a particular subject.
Expert derives from the Latin expertus (tried, tested, proven by experience). Expertus is the past participle of the verb experior (to test, to put to the test, to find out).
To paraphrase Viktor Frankl, an expert is nothing more than a person who no longer sees the forest of truth for the trees of facts. – Robert Ringer
“An expert is someone who can tell you exactly how something can’t be done.” – Peter Diamandis