A nutritionist is an expert on food, nourishment, and health. If someone is diagnosed with a condition such as diabetes, they may consult a nutritionist to help them make healthy meal plans.
While a nutritionist doesn't need to have a special license (unlike a dietitian), most of them do have advanced degrees that require them to study human health and nutrition. Anyone who's interested in eating in a more healthful way can consult a nutritionist. Nutritionists also work in sports medicine, planning hospital meals for patients, and in nursing homes. It's generally a nutritionist's job to design meals that meet certain nutritional requirements or that treat specific ailments.