毫厘 (háo lí) is a classical Chinese compound noun meaning an extremely tiny difference — literally 'a hair's breadth and a unit of length,' referring to two ancient Chinese units of measurement where 毫 is 1/10,000 of a chi (about 3.3 cm), and 厘 is 1/1,000 of a chi. Though these units are no longer used in daily life, the phrase survives idiomatically to emphasize near-imperceptible distinctions in quantity, degree, timing, or quality.

The term appears frequently in formal writing, proverbs, and cautionary statements — for example, in warnings about how small errors can lead to large consequences (e.g., '失之毫厘，谬以千里' — 'A miss is as good as a mile'). It conveys precision, sensitivity to detail, and often a sense of gravity or inevitability when discussing outcomes.