Someone you admire and look up to for whatever reason (knowledge, skills, wealth, magic powers).
It might be because of what s/he says or does or maybe because of how s/he does things.
In the course of your personal development this person might be replaced by another mentor. they might be seen as mile stones in your life as well.
You will benefit not only from their knowledge but from their life experiences.
This is a vague definition but these mentors are as colorful in personality as human beings are.