The condition that is a severe psychological disorder characterized by an irrational fear of getting fat is called anorexia .

The condition you're describing is actually known as anorexia nervosa. It is indeed a severe psychological disorder characterized by an intense, irrational fear of gaining weight, leading to a restriction of food intake and an excessive focus on body weight and shape. Individuals with anorexia often perceive themselves as overweight, even when they are underweight. This disorder can have devastating physical and emotional effects, including malnutrition and severe health complications, and it often requires comprehensive treatment involving medical, nutritional, and psychological interventions. Anorexia nervosa is part of a wider category of eating disorders, which can also include bulimia nervosa and binge-eating disorder, each with its own distinct features and challenges.