In a steam engine, a device to heat the feedwater up before it's fed into the boiler. The reason for doing this is that low-pressure exhaust steam, which can do little further work in expansion, is still fairly hot. This thermal energy would otherwise go to waste, so why not use it to heat the water going into the boiler up some? That way, less energy must be expended warming it up once it's in the boiler.
Feedwater heaters can be divided into two major categories; direct, and indirect. A direct feedwater heater mixes the hot exhaust steam directly with the cold feedwater. The heat transfer is done directly. This is obviously the efficient way to go about it, but it has a problem; the steam may have impurities in it you don't want to go into the boiler, particularly lubricating oil from the cylinders or turbine that it was used to drive. Oil in the boiler water can cause foaming, which can render the sight glasses used to monitor boiler water level inaccurate.
Therefore, indirect feedwater heaters were the first commercial successes. On steam locomotives, feedwater heaters that fall into this category include the Elesco and Coffin types.
Later, devices such as oil traps were invented to remove oil and other impurities from the exhaust steam, and the big disadvantages of the direct type were removed. Worthington Feedwater Heaters were of the direct type.
The only disadvantage of a feedwater heater is that by warming the feedwater, it makes it impossible to use an injector to get that water into the boiler. Injectors require the feedwater to be cold in order to work. A more complicated mechanical pump must be used, either a piston based pump or a turbopump. This makes a feedwater heater equipped locomotive require more maintenance than one without.