I'd have to agree with Liushka; while a Dominant is a more overarching term describing an overall personality type, a mentor, like in other aspects of life would be a teacher, a guide, someone to provide instruction. A master could also serve as a mentor, though also commanding obedience from the sub. A master would likely require a greater servitude than a mentor and a more influential component of the sub's life. It all depends upon what is desired and the relationship that is expected.