Definition 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

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