Anorexia nervosa is characterized by low body weight, inappropriate eating habits, obsession with having a thin figure, and the fear of gaining weight. It is often coupled with a distorted self image^[4][5] which may be maintained by various cognitive biases^[6] that alter how the affected individual evaluates and thinks about her or his body, food and eating.^[7] Those suffering from anorexia often view themselves as "too fat" even if they are already underweight.^[8] They may practice repetitive weighing, measuring, and mirror gazing, alongside other obsessive actions to make sure they are still thin, a common practice known as "body checking".^[9]

Anorexia nervosa most often has its onset in adolescence and is more prevalent among adolescent females than adolescent males.^[10] However, more recent studies show the onset age has decreased from an average of 13 to 17 years of age to 9 to 12.^[11] While it can affect men and women of any age, race, and socioeconomic and cultural background,^[12] anorexia nervosa occurs in ten times more females than males.^[13]

People with anorexia nervosa continue to feel hunger, but they deny themselves all but very small quantities of food.^[7] 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 similarly high mortality rates to serious psychiatric disorders.^[8] People suffering from anorexia have extremely high levels of ghrelin (the hunger hormone that signals a physiological desire for food) in their blood. The high levels of ghrelin suggests that their bodies are desperately trying to make them hungry; however, that hunger call is being suppressed, ignored, or overridden. Nevertheless, one small single-blind study found that intravenous administration of ghrelin to anorexia nervosa patients increased food intake by 12–36% over the trial period.^[14]

The term anorexia nervosa was established in 1873 by Sir William Gull, one of Queen Victoria's personal physicians.^[15] The term is of Greek origin: an- (ἀν-, prefix denoting negation) and orexe (όρεξη, "appetite"), thus meaning a lack of desire to eat.^[16] 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 appetites 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.