A mentor was a person who assisted another, less experienced, person in their professional or personal development. The recipient of such assistance was known as a mentee.
A 2152 draft biography of Henry Archer described his mathematics regarding the flux paradox as being more accurate than those of his mentors from Vulcan. (ENT: "Singularity")
Sylvia Tilly considered Michael Burnham her mentor. Burnham considered Sarek hers. (DIS: "Lethe")
According to Spock in 2269, Rayna Kapec loved Flint both as a mentor and a father. (TOS: "Requiem for Methuselah")
Beverly Crusher considered Dalen Quaice her mentor. (TNG: "Remember Me")
In 2367, Data described several acting masters as his mentors, namely Stella Adler, Constantin Stanislavsky and Garnav. (TNG: "Devil's Due")
Benjamin Sisko considered Curzon Dax to have been his mentor. (DS9: "A Man Alone", "Tears of the Prophets")
Elim Garak thought of his father, Enabran Tain, as his mentor. (DS9: "Improbable Cause", "In Purgatory's Shadow")
The Doctor considered himself the mentor of Kes. (VOY: "Darkling")
Professor Forra Gegen was described as the mentor of his assistant, Tova Veer, by Minister Odala in 2373. (VOY: "Distant Origin")
According to The Doctor in 2374, if he had had a mentor after his activation, his social experiences aboard USS Voyager would have gone more smoothly. He therefore tried to be such a mentor to Seven of Nine, whom he considered to be in a similar situation after her separation from the Borg Collective. The next year, while drunk, Seven described The Doctor as her mentor. (VOY: "Prey", "Timeless") Kathryn Janeway was similarly described as Seven's mentor by Chakotay, while calling Seven the pupil in their relationship. (VOY: "Hope and Fear")
In 2374, Vreenak described Sisko as being, among other things, a mentor. (DS9: "In the Pale Moonlight")
Later that year, Dukat described himself as the mentor of Damar. (DS9: "Tears of the Prophets")
According to Koval in 2375, Vice Admiral Fujisaki was the mentor of Luther Sloan. (DS9: "Inter Arma Enim Silent Leges")
In 2378, Captain Janeway considered Q to be in need of a mentor. When she approached Neelix about it, he agreed to deal with Q. When Janeway reminded him that Q was "no ordinary child", Neelix replied he was "no ordinary mentor". (VOY: "Q2")
In 2381, Dr. Migleemo was made Ensign D'Vana Tendi's mentor in her training as science senior officer. While he did give Tendi pointers in how to handle herself, she considered Dr. T'Ana to be her mentor, something the Counselor didn't realize. (LD: "Mining The Mind's Mines")