Today, a mentor is an experienced and trusted counselor, anyone who guides and encourages another, especially someone younger.
Mentors come in all shapes and sizes, formal and informal.
Usually your mentor is someone who takes a personal interest in your progress, seeing your potential and regarding you as just a bit more capable and talented than you think you are. Not too much more, or you'll dismiss their opinion as unrealistic. Not any less, or you'll have nothing to strive toward. We all have mentors who don't know they are our mentors. They are all around us as we watch and learn from what they do and say.