What is dieting?
Dieting is the practice of ingesting food in a regulated fashion to achieve or maintain a controlled weight. In most cases the goal is weight loss in those who are overweight or obese, but some athletes aspire to gain weight (usually in the form of muscle) and diets can also be used to maintain a stable body weight.
What is diet?
In nutrition, the diet is the sum of food consumed by a person or other organism.
Other than that....
Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. Although humans are omnivores, each culture holds some food preferences and some food taboos. Individual dietary choices may be more or less healthy. Proper nutrition requires the proper ingestion and equally important, the absorption of vitamins, minerals, and fuel in the form of carbohydrates, proteins, and fats. Dietary habits and choices play a significant role in health and mortality, and can also define cultures and play a role in religion.
However, there are many people who used the wrong "word" to mention what they actually want. For example, If they want to loose weight, they will simply say "I want to diet".. It is totally wrong.. We should say like this, " I want to get dieting" or others. There are the differences between verb and noun.
Besides that, we have to know that "dieting" doesn't means that we do not eat, but we control our eating as the definitions given above..