The word 单元 is a compound formed by combining two meaningful characters: 单 (dān), which carries the sense of "single", "individual", or "simple", and 元 (yuán), which means "unit", "element", or "basic component". Together, they create a new, integrated concept where 单 modifies 元 to specify a "single unit" or an "individual module". This term is used to describe a self-contained section within a larger system, such as a chapter in a textbook (教学单元), an apartment in a building (住宅单元), or a discrete segment of a course. Its formation exemplifies a common Chinese word-construction method where two semantic components combine logically to produce a precise, abstract noun that is more specific than either character alone.