Sometimes we find people who recognized and regarded as an expert, and some people claim they are expert.
The following is my explanation of the definition of an expert. To me, the expert is “someone who had mastered in his field of expertise, so that he can provide a very fast response (sometimes occurs without thinking – and may emerge from the unconscious) when he got stimuli associated with his expertise.”
For example, an experienced professor who teaches ‘chemical thermodynamics’ will quickly answer students’ questions (with correct answers) about the areas of his expertise without thinking hard. This professor is categorized expert in ‘chemical thermodynamics teaching.’
Perhaps, the above definition is closely related to the so-called “Skilled Memory and Expertise” proposed by Herbert Alexander Simon, David Klahr, and Kenneth Kotovsky.