In Britain, it's a pullover sweater; in America, it's a sleeveless dress worn over a blouse
[DOC] [TLE] Jumper Dress - Fashion History - LoveToKnow [PAR] A British jumper is a pullover sweater, and the British refer to a U.S. jumper as a 
... a jumper is a woman's or girl's one-piece, sleeveless, dress-length garment 
with a low round, V, or square neckline designed to wear over a blouse, sweater, 
or shirt.