This is what turns fats and oils to soap. Sodium hydroxide is also called caustic soda, or sometimes lye, although "lye" is commonly used for other compounds. All are base compounds, highly alkaline, chemically the opposite of acid.

In the soap making process, an acid is mixed with a base. Oils and fats are the acid ingredients, and lye is the base. The chemical reaction that results when the two are mixed is saponification, from the Latin for soap making.Sodium hydroxide in its present form was not around for most of soap making's history. Traditional soap used lye, in those days the liquid from leaching water through the ashes of wood, plants, and sometimes seaweed, containing sodium hydroxide, potassium hydroxide (or potash, also often called lye), carbonates, and other similar compounds. Soaps made in this manner were rough, soft, and often irritating to the skin.

An economical manner of producing sodium hydroxide was found in the sixteenth century.
The lye used to make Hummingbird soaps consists of refined sodium hydroxide dissolved
in distilled water.