A lye is an alkali metal hydroxide. Traditionally it was obtained by using rainwater to leach wood ashes, which are strongly alkaline and highly soluble in water, of their potassium hydroxide (KOH), producing lye water, a caustic basic solution. Then the lye water would either be used as such, as for curing olives before brining them, or evaporated of water to leave crystalline lye behind. "Lye" most accurately refers to sodium hydroxide (NaOH), but historically has been conflated to include other alkali materials, most notably potassium hydroxide.

Potassium hydroxide is an inorganic compound with the formula KOH, and is commonly called caustic potash.