Dieting is the practice of eating (and drinking) in a regulated fashion to achieve a particular, short-term objective. This is distinct from the more basic concept of "diet," which addresses the longer-term and more generic habit of nutritional consumption. For example, a vegan eats a diet completely without animal products, including milk; while this is a diet, it is not "dieting."

the act of restricting your food intake (or your intake of particular foods)

reducing food intake.

a waste of money