Other forms: diets, dieting
Your diet is everything you usually eat. A healthy diet, a diet high in salt, the Mediterranean diet.
The second sense is a controlled plan to lose weight — go on a diet, be on a diet — and the verb dieting means following one. The two senses are constantly confused: a diet is what you eat, and a diet is also a restriction of it. As an adjective it means low in calories: diet cola. Note dietitian, the trained professional, and dietary, the formal adjective used in phrases such as dietary requirements.
1noundietEverything a person or animal usually eats.
“Their diet is almost entirely fish.”
2nouncontrolledA controlled eating plan, usually to lose weight.
“She has been on a diet since January.”
3verborderTo eat less in order to lose weight.
“He dieted for six weeks and gave up.”
Too few real examples of this word to estimate which sense is commonest.
diet is what is eaten.
Greek diaita meant a way of life, not just food — the whole regimen a doctor prescribed. Latin and French narrowed it to eating, but the older breadth survives in the word's other life: a Diet was also an assembly, from a different Latin root about days.
Ancient Greek
δίαιτα
Latin
diaeta
Medieval Latin
dieta — regimen, regulation; assembly
Old French
diete
Middle English
diet
English, today
diet
From Middle English diet, dyet, diete, from Old French diete, from Medieval Latin dieta (“regimen, regulation; assembly”), from Latin diaeta, from Ancient Greek δίαιτα.
Etymology from Wiktionary, CC BY-SA