Final answer:
A master stream mounted to an engine is known as a deck gun or deluge gun, used particularly for firefighting applications requiring high water or foam volume delivery.
Step-by-step explanation:
A master stream that is mounted to an engine is called a deck gun or deluge gun. This device is typically used by fire services to deliver a large volume of water or firefighting foam to a fire, especially in situations where high flow is required over long distances or for combating large-scale fires like industrial fires or wildfires. The deck gun can be controlled manually or remotely and is affixed to a fire truck or engine, allowing for quick deployment and versatile use on the scene of a fire. Its effectiveness is crucial in protecting structures and controlling the spread of fires in emergency situations.