Chow-chow is a pickled relish condiment.
It is really hard to produce a standard definition for chow-chow, as it is made in several English-speaking countries, and there are regional variations even within those countries. Countries include Canada, New Zealand and the United States.
In general, chow-chow is meant to be sweet and sour. Some chow-chows can be more sour than others; some can be more hot-tasting than others, but in overall, something called a chow-chow is meant to be a bit more pungent than other pickled preserves.
In some versions, many of the ingredients are meant to come out crunchy, with distinguishable, chunky individual ingredients. Other versions have a softer texture, with the ingredients more "melted" into one another.
Typical ingredients drawn on by chow-chow recipes can include beans, bell peppers (red or green), cabbage, carrots, cucumber, corn kernels, cauliflower, onions, tomatoes (green and red), peas, etc.
The use of cabbage is more common in some parts of the southern US than it is in other parts of the US or of the world. In the American south, the primary base ingredients usually found are cabbage, onion, tomato and peppers.
In Pennsylvania and in the Canadian maritimes, the base is more likely to be green tomatoes and onion.
Typical seasonings include salt, dry mustard, celery seed, red pepper flakes, turmeric, etc.
Chow-chow is usually served as a side-garnish on plates, but occasionally you may see it as an ingredient in recipes, particularly in meat loaves, casseroles, or potato or macaroni salads, to give them some zing.
In New Zealand, you can get "chowburgers", hamburgers with a thick layer of Chow-chow on the hamburger patty. 
In America, companies such as Crown were selling bottled Chow-chow pickle by the early 1870s and before that, American customers could buy the imported, bottled Crosse & Blackwell version.
In 1877, Heinz added mustard and Chow-chow to their bottled product lines. Heinz imported his own mustard seeds and built his own mill to grind them; this dry mustard powder was then also used in the Chow-chow. He made his Chow-chow closer in sweetness to the Pennsylvania version. By 1880, it was amongst Heinz's best selling products. 
 Skrabec, Quentin R. H.J. Heinz: A Biography. Jefferson, North Carolina: McFarland, 2009. pp 72 - 73.
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