Anorexia is the medical term used to describe a condition of not eating. In fact, the word itself is made up of two Greek root words, an- meaning "not", and the verb orein which means "to eat". So quite literally, anorexia means "not to eat".
Anorexia is a broad term, used to describe failure to eat due to any cause. Perhaps the most familiar use of the term "anorexia" is actually a shortening of the true medical term anorexia nervosa, which refers to a failure to eat due to a psychological imbalance, often accompanied by an altered body image.