A good teacher betters the student's ability to learn by themselves. This involves a great many things, and sadly is contrary to the way 90% of teachers - not

A good teacher betters the student's ability to learn by themselves. This involves a great many things, and sadly is contrary to the way 90% of teachers - not just the pros - are taught to imply knowledge. That's it, and it's applicable to every situation, from Kindergarten to a personal mentor. Almost all the other comments here voice preferred methods of achieving that objective, but the objective remains.