A mental disorder where a person feels a strong desire to lose weight. Contrary to belief, when an anorexic person looks in the mirror they don't see something different. They see themselves, but in an extremely over-analyzed way, and they focus in on the fat that they have. It's not about being fat, but HAVING fat, rather. For most people with anorexia, getting rid of fat is getting rid of failure and 'becoming pure'. These people usually are perfectionists in one way or another. A lot of them also have a lot of trouble saying no, and are afraid of not pleasing those around them.
Anorexia is different from dieting. It is not self-induced. It is not caused by looking at pictures of Nicole Richie and deciding that you'd like to be glamorous and look just like her. It's a disease which is, basically, hell.