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; but while this is a diet, it is not "dieting."
The most common objective of dieting is loss of excess body fat. Some dieting is prescribed to achieve particular medical objectives, such as sodium-free diets, bland diets and soft food diets, while some dieting is actually designed to increase body fat and/or muscle weight gain.