"Eating disorder" is when a person eats, or refuses to eat, in order to satisfy a psychic need and not a physical need. The person doesn't listen to bodily signals or perhaps is not even aware of them. A normal person eats when hungry and stops eating when the body doesn't need more, when he feels the signal of satisfaction.
Eating disorders are usually classified as anorexia nervosa, bulimia nervosa or binge eating disorder in accordance with the symptoms. However, a person may have an eating disorder without belonging exactly to any of these categories.
Those who lose weight because of illness, e.g., cancer, are not considered to have an eating disorder.