A jumper is very similar to a sweater, and both words tend to be used to describe the same item. The word jumper is usually used more in the UK. A jumper is a long-sleeved item worn on the top half of your body, and like a sweater, is usually considered knitted or crocheted, but also seen made of jersey fabric or cotton too. Jumpers and sweaters can usually be dressed up or down, depending on the fabric and style of the design.
