If an individual weighs 112 pounds and that individual is size six. Based on this data if that individual eats 500 or less calories daily then yes, that individual has a eating disorder. Eating disorder is psychological disorder because when the individual goes into program of losing weight, he/she with the passage of time may get into habit of eating less than required. This usually starts when an individual sees himself/ herself as becoming popular by losing weight ( yes, when you are losing weight people around you start noticing your body change and you start to feel proud). This psychologically starts to create a goal in the mind of the individual that people are admiring him/ her, so that pride should not fall at any cost and that's the point where one falls into eating disorder.
By the way don't confuse yourself "eating disorder" with "selective disorder" both are different.