Source: Vintage Recipe Information from Old Time Pickling and Spicing Recipes - Florence Brobeck (c. 1953)
A relish is a comparatively simple mix-
ture of chopped vegetables; should be, by most recipes,
finelychopped. But with all mixtures in the general group
of pickling and spicing, the term "relish" is used loosely.
Generally, it means any spicy sour or sweet mixture served
as garnish or to spice up another food such as an aspic, cold
meats, sandwiches, or to add color and flavor to a serving
of meat. Its color is as important as its flavor in creating
appetite appeal- a spoonful of green pickle relish on a
sandwich plate, a spoonful of beet relish added to the cold
meat platter-these tell the hungry family that good flavor
is theirs, and contrast and variety, too.