The Chinese word **眼线 (yǎnxiàn)** is a compound noun formed by combining the character **眼 (yǎn)**, meaning "eye," with **线 (xiàn)**, meaning "line" or "thread." Literally, it translates to "eye line," which descriptively refers to a line applied around the eye. This formation follows a common structural pattern in Chinese where two concrete nouns are combined to create a new term with a specific, often metaphorical, meaning. In its primary modern usage, it directly denotes **eyeliner** (the cosmetic). By metaphorical extension, it also means an **informant** or "spy" (one who watches and reports, forming a "line" of intelligence back to someone, much like the cosmetic defines the eye's line). Thus, the word's formation logically progresses from a literal physical description to its practical application and then to a figurative social role.