A sweater is a knitted piece of clothing that covers the top part of your body. Some people have ugly Christmas sweater contests—and if you wear your neon green reindeer sweater covered with pom-poms, you just might win!
In Britain it's a jumper, but in most of North America, it's a sweater. You put on a sweater either by pulling it over your head (if it's a pullover), or by putting your arms in the sleeves and shrugging it onto your shoulders (if it's a cardigan). As you might have guessed, sweater comes from sweat—the earliest sweaters were worn by rowers, and were intended "to produce sweating and reduce weight."