"Nutrition" ultimately comes from the Latin verb "nutrire", which means to feed or nourish. From the same word we get "nutrient", that is, an element that feeds or nourishes us.
A person with poor nutrition eats poorly, either because of bad food choices or because he does not have the money to buy nutritious food.
Whole grain flour is more nutritious than white flour because it contains all the protein, vitamins, and minerals of a natural versus a refined product.
A person with malnutrition is suffering from the physical signs of lacking essential nutrients.
A person who studies nutrition investigates how the body uses certain substances for health and growth.