- A substance which dissolves in water, thus forming a lather, and is used as a cleansing agent. Soap is produced by combining fats or oils with alkalies or alkaline earths, usually by boiling, and consists of salts of sodium, potassium, etc., with the fatty acids (oleic, stearic, palmitic, etc.). See the Note below, and cf. Saponification. By extension, any compound of similar composition or properties, whether used as a cleaning agent or not.