The word 单子 is formed by combining the character 单, which carries the core meaning of a list, record, or bill, with the character 子, which here functions not with its literal meaning of "child" but as a common suffix that nominalizes and concretizes the preceding concept. This fusion transforms the abstract idea of a "list" or "single item" into a tangible, specific object, resulting in a compound that refers to a physical or practical document, such as a prescription slip, a receipt, or an order form.