What is the medical terminology of weight loss?

There may be an obscure medical term of Latin/Greek origin, but everybody just uses "weight loss."

The concept of weight loss is very general, nondescript, and can be caused by a number of unrelated etiologies such as negative metabolic energy balance (e.g., anorexia, cachexia, diet change, hyperthyroidism, adrenal cortical disease, gastrointestinal malabsorptive diseases such as lymphangiectasia), dehydration, surgical excision of tissue, etc.

Weight loss associated with negative energy balance is not a disease in itself and is only a symptom/clinical sign that is representative of a normal or abnormal physiologic response to a stressor put on the body.