Anorexia nervosa is an eating disorder characterized by an obsessive fear of gaining weight. The terms anorexia nervosa and anorexia are often used interchangeably, however anorexia is simply a medical term for lack of appetite. Anorexia nervosa has many complicated implications and may be thought of as a lifelong illness that may never be truly cured, but only managed over time.

Anorexia nervosa is often coupled with a distorted self image which may be maintained by various cognitive biases that alter how the affected individual evaluates and thinks about her or his body, food and eating. Persons with anorexia nervosa continue to feel hunger, but deny themselves all but very small quantities of food. The average caloric intake of a person with anorexia nervosa is 600–800 calories per day, but extreme cases of complete self-starvation are known. It is a serious mental illness with a high incidence of comorbidity and the highest mortality rate of any psychiatric disorder.

Anorexia most often has its onset in adolescence and is most prevalent among adolescent girls. However, more recent studies show that the onset age of anorexia decreased from an average of 13 to 17 years of age to 9 to 12. While it can affect men and women of any age, race, and socioeconomic and cultural background, Anorexia nervosa occurs in females 10 times more than in males. While anorexia nervosa is quite commonly (in lay circles) believed to be a woman 's illness, it should not be forgotten than ten per cent of people with anorexia nervosa are male.

The term anorexia nervosa was established in 1873 by Sir William Gull, one of Queen Victoria's personal physicians. The term is of Greek origin: an- (ἀν-, prefix denoting negation) and orexis (ὄρεξις, "appetite"), thus meaning a lack of desire to eat. However, while the term "anorexia nervosa" literally means "neurotic loss of appetite" the literal meaning of the term is somewhat misleading. Many anorexics do enjoy eating and have certainly not lost their appetite as the term "loss of appetite" is normally understood; it is better to regard anorexia nervosa as a self-punitive addiction to fasting, rather than a literal loss of appetite.

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