"Mentoring is a lifelong relationship, in which a mentor helps a protege reach her or his God-given potential" -- Bob Biehl in Mentoring: Confidence in Finding a Mentor and Becoming One.
"Mentoring is a relational experience through which one person empowers another by sharing God-given resources." -- Paul Stanley and Robert Clinton in Connecting: The Mentoring Relationships You Need to Succeed. They also provide an expanded definition: "Mentoring is a relational process in which a mentor, who knows or has experienced something, transfers that something (resources of wisdom, information, experience, confidence, insight, relationships, status, etc.) to a mentoree, at an appropriate time and manner, so that it facilitates development or empowerment." These authors actually look at seven different types of mentoring relationships which go from very informal to highly formal.
Merriam-Webster defines a mentor as "a trusted counselor or a guide."
The above are what most refer to as the "traditional" view of mentoring, where an older experienced person takes a younger person under their wing and helps them along by sharing insights and experience. It's a one-to-one relationship, and may last for quite a long time.
In Part Two of "What is Mentoring?" we'll take a look at some approaches more popular today which contrast with traditional mentoring.