Yes, diet is a Scrabble word!

Verb

1. Follow a regimen or a diet, as for health reasons

2. Eat sparingly, for health reasons or to lose weight

Noun

1. A prescribed selection of foods

2. A legislative assembly in certain countries (e.g., Japan)

3. The usual food and drink consumed by an organism (person or animal)

4. The act of restricting your food intake (or your intake of particular foods)