One of those strange and distinctly American concoctions that stretch the meaning of the word salad (not to be confused with word salad). Waldorf Salad was invented by Oscar Tschirky at the Waldorf-Astoria hotel in New York in the 1890s.
Proportions and even ingredients vary widely from one recipe to another; I've tried to suggest the general range here:
a tablespoon or so of sugar or honey
- 3 or 4 apples (tart eating apples, or a mixture of tart and sweet ones), cored and diced and sprinkled with lemon juice so they don't turn brown
- 1 or 2 stalks of celery, diced
- 1/3 cup of coarsely chopped walnuts
- 1/3 cup of raisins, optionally plumped, or halved seedless fresh grapes
- 1/3 cup of any of the following:
- mayonnaise (if you're a Waldorf Salad traditionalist)
- yogurt (if you hate mayonnaise and/or want to lower the fat content)
- sour cream (if you hate mayonnaise and don't care a flying fig about the fat content)
Mix all of the above until the solid ingredients are evenly coated with, and loosely stuck together by, the mayonnaise or yogurt or sour cream. Either immediately or after chilling for a while, serve on lettuce.
Ingredients can be added, omitted, augmented, diminished, or substituted more or less at will. Apparently the original version had only apples, celery, and mayonnaise (plus, of course, the lemon juice). I cannot understand why anyone would even think of eating this stuff without walnuts in it. Versions also exist that contain turkey, cranberries (hmm... Thanksgiving leftovers, perhaps?), tuna, Jell-O, blue cheese, mustard, crab, and pambů ví what else.
Edit: And so does momomom, who mentions a dried cranberry variant (really old Thanksgiving leftovers, perhaps?). Thanks also to anthropod, for pointing one of my links in the right direction.
Further addendum: LudditeAndroid reports that (at least) in Iowa, Waldorf salad is served as a dessert, and made with vanilla or chocolate whipped cream in place of the mayonnaise/yogurt/sour cream.
Still more Waldorfiana: Shaogo says that a version with grapes is served as an appetizer to Korean meals, and notes the similarity of the Iowan variant to ambrosia.