A wiper is the most junior crewmember in the engine room of a ship. The role of a wiper consists of cleaning the engine spaces and machinery, and assisting the engineers as directed.
In the United States Merchant Marine, in order to be occupied as a wiper a person has to have a Merchant Mariner's Document and STCW certificate issued by the United States Coast Guard. Because of international conventions and agreements, all wipers who sail internationally are similarly documented by their respective countries.
Railroad workers who performed similar jobs were also known as wipers, or in the UK as 'cleaners'.