Final answer:
A steam engine is a type of heat engine that converts thermal energy into mechanical energy, utilizing the principles of heat transfer and thermodynamics.
Step-by-step explanation:
A steam engine is a classic example of a heat engine, which operates on the principle of converting thermal energy into mechanical energy. This conversion is fundamental to the operation of steam engines found in old-style trains and steam turbines in power plants. Specifically, in a steam engine, water is heated until it becomes steam. The high-pressure steam then drives a turbine or pistons that perform mechanical work, such as moving a train or driving electrical generators.
The steam engine is an application of the second law of thermodynamics, which among other things addresses the efficiency of heat transfer when performing work. A steam engine uses the energy from heat - often produced by burning coal or other fuels - to generate mechanical motion. This is not a 100% efficient process, as some energy is always lost as waste heat to the surroundings, but it is an effective method of converting heat to work.
The answer to the fill-in-the-blank part of the question is 'heat engine'. Therefore, a steam engine is a type of heat engine.