Latin from Greek Mentōr,  the advisor of young Telemachus in Homer's Odyssey.

True mentors are rare.  You are lucky to have one or two in your life. 

A special teacher may evolve into a advisor, then mentor, then colleague and friend.  Time spent with a special teacher may just be a single semester or many years.  In any case, the special ones are with you for your life.  "You are who you are because of your teachers.  Your teachers are who they are because of their teachers.  Thus their teachers are also your teachers and so on."  This genealogy forms the foundation of one's intellectual identity.