A tutor is an instructor who gives private lessons. The most famous example of a tutor is Aristotle, who tutored Alexander the Great. A tutor is not to be confused with a teacher who is employed in the education of groups. To tutor (as a verb) is to perform the functions of a tutor. Shadow education is a pejorative name for private supplementary tutoring that is offered outside the mainstream education system. Private tutoring can help high achievers reach new levels.