What is the proper definition of an engine?

The definition of an engine is best captured by stating that it is a machine designed to convert energy into useful mechanical motion. This description highlights the engine's primary function: transforming various forms of energy—such as chemical, thermal, or electrical—into kinetic energy that can be used to perform work, such as moving a vehicle, operating machinery, or driving generators.

Engines can operate on a variety of energy sources, including fuels like gasoline or diesel, or through electrical inputs, and the focus on useful mechanical motion underscores their role in facilitating movement or mechanical processes. This definition is comprehensive and encompasses the fundamental purpose of what engines do in practical applications, such as powering cars, airplanes, and industrial machines.