A Side Dish is essentially any food served on the side of the plate or to the side of the main plate. An easy way to think of them is that Side Dishes are accompaniments and not the main course. Side Dishes are typically smaller offerings than the showpiece, main dish of the meal.
The purpose of a Side Dish is to balance out, round out or in other ways compliment the main dishes of a meal. In some cases this may mean to provide a mellowing flavor, in others a bit of spice. Side Dishes can also be used in conjunction with the main meal, to soak up rich sauces for example, or as a “bed” for the main dish to rest upon.
Side Dishes also fill out the offering of the main dish and are a way to guarantee to slake the hunger of all but the most famished diners. Side Dishes can also deliver vitamins missing in the entrée creating a perfectly balanced meal. Salads and Vegetable dishes come to mind in this regard. Finally, Side Dishes often preserve tradition, as in the family dishes many clans enjoy on holidays like Thanksgiving, Christmas or Passover.
What we are trying to say, is that past the formal definition, a Side Dish can vary widely, from simple raw or steamed vegetables to elaborate quiches, soups, potato dishes and just about anything at all.