Coveralls are a one-piece work suit — trousers and a top joined together, usually with long sleeves and a zip or buttons up the front. You put them on so paint, oil, dirt, or dust does not ruin the clothes underneath. Mechanics, painters, factory workers, and farmers wear them. In American English this is the usual word. British speakers more often say overalls or boiler suit for the same full suit, which is confusing, because in American English overalls are a different thing: trousers with a front flap and shoulder straps, and no sleeves. Though actually, people do not always wear coveralls over other clothes. Many workers wear them as the work clothes themselves, with just a T-shirt underneath. The word is plural in form, like trousers: we say a pair of coveralls even though it is one piece.