线索 (xiànsuǒ) literally combines 线 (xiàn), meaning 'thread' or 'line', and 索 (suǒ), meaning 'to search' or 'rope'. Together, they evoke the image of following a thread to find something — hence its core meaning: 'clue' or 'lead' in an investigation, puzzle, or mystery. It’s commonly used in contexts involving deduction, detection, or problem-solving, such as police work, scientific research, or even everyday reasoning.

Unlike more casual terms like 点子 (diǎnzi, 'idea') or 办法 (bànfǎ, 'solution'), 线索 carries a formal, investigative connotation — it refers specifically to a piece of information that helps uncover a larger truth or connect scattered facts. It’s often paired with verbs like 发现 (fāxiàn, 'to discover'), 提供 (tígōng, 'to provide'), or 追踪 (zhuīzōng, 'to track'). While not limited to crime stories, it appears frequently in detective novels, news reports about investigations, and academic discussions where evidence is being pieced together.