Read the dictionary definition of diet. All definitions for this word.
1. the act of restricting your food intake (or your intake of particular foods)
2. the usual food and drink consumed by an organism (person or animal)
3. a prescribed selection of foods
4. a legislative assembly in certain countries (e.g., Japan)
5. eat sparingly, for health reasons or to lose weight
6. follow a regimen or a diet, as for health reasons
1. He has high blood pressure and must stick to a low-salt diet
See also:
Merriam-Webster definition of diet