The traditional definition of an 'expert' is someone who can help the client to deal practically with difficulties in the real world - who has specialised knowledge of a particular aspect of that world, and therefore is able to identify and help to resolve the practical difficulties that arise from the client's lack of expertise in that particular field of endeavour. By virtue of the fact that he knows more about a particular subject than the person who is asking for help, he (or she) therefore has the power to resolve the client's difficulties - power that the client doesn't possess. That's what the client pays for. You go to a dentist, for example, and you ask for his expert, professional help. You have pain that you can't do anything about, but he knows about teeth and he can do things to your teeth that you can't do. So we can define an expert as someone who has a practical set of skills which you need but don't intrinsically possess.