The primary ingredient in a Waldorf salad is apples. Traditionally, a Waldorf salad consists of diced apples mixed with celery, walnuts, and mayonnaise. This dish originated from the Waldorf Astoria Hotel in New York City in the late 19th century and has since evolved with various variations, including the addition of grapes or dried fruits. The combination of the crunchy texture of celery and walnuts with the sweetness of apples creates a refreshing and delightful salad that can be served as a side dish or light meal.