A nanny is a full-time babysitter, someone whose job is taking care of a family's children. If you're a lucky kid, your nanny will be like Mary Poppins or Maria from "A Sound of Music."
Being a nanny goes beyond hourly childcare — a nanny is usually the person who spends the most time with a baby or child. A nanny might feed, bathe, play with, and otherwise care for a toddler, or drive an older child to ballet lessons. Most nannies work in the child's home, sometimes even living there. A completely different definition of nanny is "female goat." The word's origin is probably as a nickname for Ann, a generic woman's name, though it's also traditionally used for "close female adult," such as an aunt.