A substance used to enhance or complement flavour of food, such as mustard, ketchup, salt, or spices.
It’s usually added to prepared food. For example sprinkling Parmesan cheese on your pasta or adding salad dressing to a salad or adding relish to your hot dog or adding pickles and mayonnaise to your hamburger.
I think the list is endless. Think of all the things you can add to prepared food to enhance flavour.
Other examples: soy sauce, wasabi sauce, pickles, BBQ sauce, ranch dressing, freshly ground pepper, horseradish sauce or chilli pepper.