A crayfish is a small animal like a little lobster that lives in rivers, streams, and lakes, not in the deep sea. People also say crayfish for the meat. It has a hard shell, two claws, and a tail that folds under. British and general English use crayfish; Americans, especially in the South, often say crawfish for the same animal. Australia has yabbies, which are a kind of crayfish. Neutral, everyday when you are talking about food or nature; not slang and not formal. Though actually, the -fish at the end is a folk story: English speakers took an old French word (crevice; today’s French is écrevisse) and reshaped it to sound like fish, because it lives in water and you eat it. It is not a fish. Scientists may say freshwater lobster; cooks and menus just say crayfish.