A motivator is a person or a thing that makes you want to do something, or to keep going until you finish. A person: a coach, a teacher, a friend who will not let you quit. A thing: money, fear of failing, a poster on the wall, the thought of your family. In offices and self-help talk the word is very common; schools use it too. It is a little more formal than “the reason I do it,” but not fancy. Though actually, people stretch it: they call someone “a great motivator” as praise, even when that person is really just cheerful. And one person’s motivator (competition, say) is another person’s stress. The word does not tell you whether the push is kind.