What is the term for an assembly that includes moving parts completing a functional motion?

The term that describes an assembly with moving parts that perform a functional motion is known as a mechanism. Mechanisms are fundamental components in engineering and design, as they convert forces and motions into desired actions. They often consist of various parts connected in such a way that allows them to work together to create movement, efficiency, or force amplification, which is crucial in the design of machines and mechanical devices.

In contrast, a device is a more general term that can refer to any physical item designed for a specific purpose but does not necessarily encompass the idea of moving parts or motion. An apparatus typically refers to a complex machine or equipment designed for a specific function, which may include mechanisms but isn't synonymous with them. A module often signifies a self-contained unit that can be part of a larger system, but like a device and apparatus, it does not inherently imply the presence of moving parts or the specific functionality of producing motion.

Thus, "mechanism" is the most precise term among the choices provided, as it specifically denotes the assembly of parts that create motion.