Anorexia is a decreased appetite. While the term in non-scientific publications is often used interchangeably with anorexia nervosa, many possible causes exist for a decreased appetite, some of which may be harmless, while others indicate a serious clinical condition or pose a significant risk.
Anorexia is a symptom, not a diagnosis. When a healthcare provider states that a patient has anorexia, they are simply referring to a decreased appetite. This means that the provider must find and treat the underlying cause of the anorexia. Anorexia is not to be confused with the mental health diagnosis, anorexia nervosa. Because the terms are commonly used interchangeably, a provider must clarify to a patient to which they are referring as to avoid confusion. Anyone can manifest with anorexia regardless of their gender, age, or weight.
The term is from Ancient Greek ανορεξία: 'ἀν-' "without" + 'όρεξις', spelled 'órexis' meaning "appetite".
Anorexia (symptom) simply manifests as a decreased appetite. This can present as not feeling hungry or lacking the desire to eat. Sometimes people do not even notice they lack an appetite until they begin to lose weight from eating less. In other cases, it can be more noticeable, such as when a person becomes nauseated from just the thought of eating. Any form of decreased appetite that leads to changes in the body (such as weight loss, muscle loss) that is not done intentionally as part of dieting is clinically significant.
Physiology of Anorexia
Appetite stimulation and suppression is a complex process involving many different parts of the brain and body by the use of various hormones and signals. Appetite is thought to be stimulated by interplay between peripheral signals to the brain (taste, smell, sight, gut hormones) as well as the balance of neurotransmitters and neuropeptides in the hypothalamus. Examples of these signals or hormones include neuropeptide Y, Leptin, Grehlin, insulin, serotonin, and orexins/hypocretins. Anything that causes an imbalance of these signals or hormones can lead to the symptom of anorexia. While it is known that these signals and hormones help control appetite, the complicated mechanisms regarding a pathological increase or decrease in appetite is still being explored
During the post-operative recovery period for a tonsillectomy or adenoidectomy, it is common for adult patients to experience a lack of appetite until their throat significantly heals (usually 10–14 days).
Anorexia is a relatively common condition that can lead patients to have dangerous electrolyte imbalances, leading to acquired long QT syndrome which can result in sudden cardiac death. This can develop over a prolonged period of time, and the risk is further heightened when feeding resumes after a period of abstaining from consumption.
Care must be taken when a patient begins to eat after prolonged starvation to avoid the potentially fatal complications of refeeding syndrome. The initial signs of refeeding syndrome are minimal but can rapidly progress to death. Thus, the reinitiation of food or oral intake is usually started slowly and requires close observation under supervision by trained healthcare professionals. This is usually done in a hospital or nutritional rehabilitation center. 
Anorexia can be treated with the help of orexigenic drugs.
Anorexia vs Anorexic vs Anorexia Nervosa
Example of severe anorexia leading to an anorexic appearance
Anorexia is a medical symptom defined as a decreased appetite.
Anorexic is a description of somebody with the stereotypical thin, frail, malnourished appearance. The appearance is classically associated with anorexia, although in rare cases do patients end up becoming anorexic. An anorexic or anorectic is also a description given to substances that cause anorexia for weight loss purposes.
Anorexia nervosa is an eating disorder characterized by food restriction due to the strong desire to remain thin. It is considered a mental health diagnosis where people see themselves as overweight regardless of their weight or appearance. The person does not necessarily exhibit anorexia as a symptom in their quest to restrict food intake.