What is a group of parts that work together as a whole called?

The term that best describes a group of parts that work together as a whole is "system." In scientific contexts, a system refers to an organized group of interrelated components that interact with one another to function as a single entity. Systems can be found in various domains, including biology, physics, and engineering; for example, the human body is a biological system made up of various organs that work together to maintain life.

In contrast, while "collective," "assembly," and "unit" can imply a grouping of parts or components, they do not necessarily emphasize the interaction and functioning aspect as strongly as "system" does. A collective may refer to a group without implying functional interdependence, and an assembly typically denotes a gathering of items rather than the cohesive functioning of individual components. A unit suggests a single entity or component that may not necessarily infer interaction with other components. Therefore, "system" is the most accurate and encompassing term for describing interconnected parts that collectively perform a function.