线索 (xiànsuǒ) means 'clue' in investigations or 'thread' of a story — it's the abstract connection that helps solve a problem or follow a narrative. Compare with 踪迹 (zōngjì), which refers to literal physical tracks or footprints; and 线 (xiàn), the literal word for thread or string. 线索 is always metaphorical for 'lead' or 'narrative thread'.