lye, name commonly used for a strongly alkaline solution. It originally meant a solution of potassium carbonate (potash) prepared by leaching wood ashes with water, but now the name also means a solution of sodium hydroxide or potassium hydroxide. Lye is also used to refer to the undissolved solute. Common household lye is usually sodium hydroxide. Lye should be used with caution as it is caustic and poisonous.