'辅助线' literally means 'assisting line' — combining 辅 (fǔ, to assist or support), 助 (zhù, to help), and 线 (xiàn, line). It refers specifically to non-final, temporary lines drawn in technical drawing, graphic design, architecture, or geometry to guide accurate placement, alignment, proportion, or construction of main elements. These lines are typically light, dashed, or faintly sketched and are erased or hidden before final output.

Unlike decorative or structural lines, 辅助线 serve purely functional, planning-oriented roles — for example, establishing vanishing points in perspective drawing, marking centerlines in mechanical blueprints, or constructing geometric proofs. The term is standard in Chinese design education, CAD software interfaces (e.g., AutoCAD, SketchUp), and mathematics textbooks. It carries a neutral, professional register and is rarely used outside visual or technical contexts.