A Relish generally used to be a pungent, tangy, chunky pickled condiment served in small dishes, of which you placed a bit at the side of your plate to add zip to a meal, especially anything your mother had cooked.
It is both spoonable and spreadable, finely or coarsely chopped, with a bit of liquid in it. It is not a purée — no matter how finely it is chopped, you can see each piece in it. Nor is it meant to be poured like a sauce.
The United States Department of Agriculture uses this definition: “Relishes are made from chopped fruits and vegetables that are cooked with seasonings and vinegar.” 
It has come to mean a lot more, though. Relishes can be sweet or tangy or both. And probably more relish is put on hot-dogs and hamburgers than now reaches the side of plates.
Literature & Lore
“Now I begin to Relish thy advice.” — Shakespeare.
 United States Department of Agriculture (USDA). 2009. Complete guide to home canning. Agriculture information bulletin No. 539. Available at: http://nchfp.uga.edu/publications/publications_usda.html (Accessed January 2015). Page 1-27.