Definitions of diet:

• noun:   the act of restricting your food intake (or your intake of particular foods)
• noun:   the usual food and drink consumed by an organism (person or animal)
• noun:   a prescribed selection of foods
• noun:   a legislative assembly in certain countries (e.g., Japan)
• verb:   eat sparingly, for health reasons or to lose weight
• verb:   follow a regimen or a diet, as for health reasons

  Example: "He has high blood pressure and must stick to a low-salt diet"