"Mechanical device" refers to any physical machine that operates by principles of mechanics. There are various synonyms used to describe such devices, depending on their specific purpose, shape, and size. Some commonly used synonyms for mechanical device include engine, apparatus, appliance, gear, machinery, instrument, mechanism, contraption, and gadget. An engine is a mechanical device that converts energy into motion. A contraption usually refers to a complex yet ingenious mechanical device designed for a specific purpose. A gadget is a small mechanical device that performs a simple task, often used for convenience. Overall, the term "mechanical device" encompasses a wide range of machines that play a critical role in our day-to-day lives.