石灰 (shí huī) is a chemical compound—calcium oxide (CaO)—produced by heating limestone. The character 石 means 'stone', reflecting its mineral origin, while 灰 means 'ash' or 'powder', describing its fine, white, powdery appearance after processing. Together, they literally convey 'stone ash', capturing both its source and physical form.

This word is used primarily in industrial, agricultural, and construction contexts—for example, to adjust soil pH in farming, as a disinfectant in animal husbandry, or as a component in traditional mortar and plaster. It’s distinct from 'limestone' (石灰石, shí huī shí), which is the raw rock, and 'slaked lime' (熟石灰, shú shí huī), the hydrated form used in building. In daily speech, 石灰 rarely appears outside technical or rural settings, and learners should not confuse it with the unrelated homophone 'lime' (the fruit, 柠檬, níng méng).