A dieter is someone who is trying to become thinner by controlling what they eat. They may count what goes on the plate, skip foods they like, or follow a plan from a book or a phone. The word is everyday and a little informal, and the point is weight, not “eating well” in general. A person who chooses vegetables because they want to be healthy, but is not trying to get smaller, is not usually called a dieter. Though actually, people stretch it as a joke: “I’m a dieter this month” can mean a short no-sugar week, not a new identity. It is simply diet plus -er: the person who does the dieting. It is not a job title, and it is not the same as the trained food expert.