This is the 4631st most frequent Russian word.
диета
Diet.
Here, 'диета' is used to mean a structured plan of eating aimed at achieving a health-related goal.
Я начал новую диету, чтобы улучшить своё здоровье.
I started a new diet to improve my health.
Here, 'диете' (prepositional case of 'диета') implies the general concept of regulating one's diet, often for health or weight management.
После праздников многие задумываются о диете.
After the holidays, many people think about dieting.
Here, 'диету' (accusative case of 'диета') refers to a medically prescribed set of dietary rules for health improvement.