So I'm doing a challenge where my friend says I cant eat the salad dressing ,as it like a junk food and junk food = fast food but since there's a difference between them. I wanted to know if salad dressing is a fast food or not.
I don't think junk food and fast food are always interchangeable. Some fast foods are healthy (salads, for example). I can't think of any right now, but I imagine there are foods that are made slowly but still have little nutritional value.
Anyway, focusing on salad dressing, I would not primarily consider this "fast food". Yes, it's "fast" in the sense that it's already made (assuming you're talking about ones you buy at the store) and it's "food" in the sense that you can eat it. But fast food is commonly associated with pre-cooked meals, pre-made ingredients, and mass-production. The typical example would be a burger.
I would say that salad dressing is a condiment:
A condiment or table sauce is a spice, sauce, or preparation (such as onions) that is added to food to impart a specific flavor, to enhance the flavor, or, in some cultures, to complement the dish. The term originally described pickled or preserved foods, but its meaning has changed over time.