细线 literally means 'fine thread' — combining 细 (xì), meaning 'thin', 'fine', or 'delicate', and 线 (xiàn), meaning 'thread', 'line', or 'wire'. Together, they describe something slender, flexible, and often fragile, like a single strand of silk, nylon, or even spider silk. While it can refer to literal threads used in sewing or crafting, it’s frequently used in biological or zoological contexts to describe extremely thin natural structures.

In everyday usage, 细线 appears in descriptions of animal anatomy (e.g., the fine threads spiders use to spin webs) or delicate man-made objects like fishing lines or surgical sutures. It emphasizes not just thinness but also continuity and tensile quality — the object is long, narrow, and usually taut or functional. Unlike broader terms like 线 alone, 细线 carries an implicit contrast with thicker or coarser alternatives, making it useful for precise visual or tactile description.