caustic, any strongly corrosive chemical substance, especially one that attacks organic matter. A caustic alkali is a metal hydroxide, especially that of an alkali metal; caustic soda is sodium hydroxide, and caustic potash is potassium hydroxide. Silver nitrate is another caustic substance; it is sometimes called lunar caustic. Most inorganic acids, e.g., sulfuric acid, are caustic, especially when concentrated.
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