A tutor was someone who aided a student with their schoolwork.
The Kriosian empathic metamorph Kamala was surrounded by servants and tutors during her childhood. (TNG: "The Perfect Mate")
Joshua Albert had a tutor prior to applying for Starfleet Academy. (TNG: "The First Duty")
In 2366, after Geordi La Forge and Christy Henshaw left together, Worf explained to Data that he'd been tutoring him in the ways of love, saying he learned very quickly. (TNG: "Transfigurations")
In 2377, Icheb tried to be Naomi Wildman's tutor in genetics aboard the USS Voyager, but she was more interested in playing with the DNA jigsaw puzzle he brought her than learning with it. (VOY: "Shattered")