An entrée salad invented in San Francisco around 1905. It is topped by a generous serving of Dungeness crab meat, and features a distinctive horseradish-forward dressing. It was very popular for decades but is now rarely seen. Dungeness is not always available so other types of crab are often substituted. As are shrimp, or even lobster. Google "lump crab" to find frozen or canned alternatives.
There are many riffs on the Louie, including abominations using Thousand Island dressing or artificial crab. But this one holds true to tradition, is fun to make, looks spectacular, and is a thrill to eat.