Sometimes we find people who recognized and regarded as an expert, and there are also people who claim they are expert.
This is my explanation on definition of expert. To me, the expert is “someone who has mastered in his field of expertise, so that he can provide 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 teach ‘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.