The real definition of junk food (or, of any of its synonyms) should recognize the fact that the adjective is applied exclusively to food items that children, and especially teenagers, find appetizing. Thus, codliver oil, despite its undeniable greasiness and artificially added vitamins and preservatives, is not junk food, because children loath it. Cake, which children love, is, on the other hand, a non-basic (or junk) food, despite containing flour, eggs, milk products, fruit, and sugar (which, with the inexplicable exception of the sugar, are all individually classed as "basic" food items).
Another factor which distinguishes "junk" from "basic" (or "nutritious" food), is the amount of effort the lady of the house expends on preparing that food.
I'm not sure this is true, but it's at least plausible.