A crawfish is the same animal as a crayfish: a small freshwater cousin of the lobster, with a shell, claws, and a tail people eat. Crawfish is the American word, especially in the US South — Louisiana crawfish boils are the picture many Americans have. In most other English, people say crayfish. It is not a kind of fish, even though the spelling looks that way. Cooks and menus use it for the animal and for the meat. Informal to neutral; regional, not posh. Though actually, Americans also use crawfish as a verb: to crawfish (out) is to back away from a promise, because the animal shoots backwards in the water. That use is informal and Southern. In Taiwan you will meet the animal as 小龍蝦, often spicy, and that is usually this same creature.