In Britain, it's a pullover sweater; in America, it's a sleeveless dress worn over a blouse
coat / overcoat (also raincoat) = what you wear in winter over your ... A 
jumper / sweater / pullover is often made of wool and you wear it over a shirt ... A t
-shirt is a short-sleeved (or sleeveless) cotton shirt for summer, or for under your 
shirt.