"Junk food" generally refers to foods that contribute lots of calories but little nutritional value. Of course, what's considered "junk food" depends on whom you ask. Some might say pizza is junk food, for example. But I personally don't think so, since it contributes real food with nutrients, like cheese and tomato sauce. Add whole-wheat or part whole-wheat crust, plus veggies as a topping, and I'd say pizza completely exits the junk food category.

By Wikipedia, junk food is of little nutritional value and often high in fat, sugar, salt, and calories. However, how little value it is? How high fat it is? There is not a clearly definition for junk food .