Clean eating is more of a lifestyle than just a short-term diet. The main premise is that "clean" food is whole, unrefined food without additives or preservatives, artificial flavoring, artificial coloring and sugar substitutes. Nothing is off-limits as long as it's made of pure, unadulterated ingredients.
Those who practice clean eating focus on choosing foods with less than four to six ingredients, all of which should be easily recognizable—nothing you can't pronounce. It also usually involves exercising regularly, drinking plenty of water, and ensuring proper nutrition through supplements.