Piccalilli is a British relish of chopped pickled vegetables and spices. British piccalilli contains various vegetables - invariably cauliflower and vegetable marrow - and seasonings of mustard and turmeric. It is used as an accompaniment to foods such as sausages, bacon, eggs, toast, cheese, and tomatoes.
In the Northeastern United States, commercial piccalillis are based on diced sweet peppers, either red or green. This style is very similar to sweet pepper relish, with the piccalilli being distinguished by having a darker red or green color and like British piccalilli, the chunks are larger and it is tangier and less sweet. It is a popular topping on such foods as hamburgers and hot dogs.
In the Midwestern United States, commercial piccalillis are based on finely chopped gherkins; bright green and on the sweet side, they are often used as a condiment for Chicago-style hot dogs. This style is sometimes called "neon relish."
In the Southern United States, piccalilli is less popular. In its place, chow-chow, a relish with a base of chopped green (unripe) tomatoes is offered. This relish may also include onions, bell peppers, cabbage, green beans and other vegetables. While not exactly similar to other piccalillis, chow-chow is often called as such and the terms may be used interchangeably.