Meaning of “diet” - Learner’s Dictionary

diet

noun uk us /daɪət/
USUAL FOOD [ C, U ]

B1 the type of food that someone usually eats:

His diet isn't very healthy.
SPECIAL FOOD [ C ]

B1 a period when someone eats less food, or only particular types of food, because they want to become thinner, or because they are sick:

No cake for me, thanks - I'm on a diet.

(Definition of “diet noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)