On a steam locomotive, a raised, domed protrusion in the top of the boiler. Being the highest point in the boiler, steam is guaranteed to be present here. Therefore, the steam is always drawn from this point; the dry pipe that carries the saturated steam either directly to the cylinders (or, in a superheated locomotive, to the superheater) begins here. Conventionally, the throttle is located here, at the entrance to the dry pipe, but many modern locomotives have a front-end throttle.
Early locomotives generally had very large steam domes, but as the development of the steam locomotive progressed, steam domes shrank. This was partly due to clearance issues; a larger steam dome would not fit within the loading gauge as the size of the boiler grew. It also became clearer that a large steam dome was not required. Indeed, some later locomotives, for example the New York Central's Niagara, were completely domeless.