Soap is an anionic surfactant used in conjunction with water for washing and cleaning, which historically comes either in solid bars or in the form of a viscous liquid.

Soap consists of sodium or potassium salts of fatty acids and is obtained by reacting common oils or fats with a strong alkaline solution (the base, popularly referred to as lye) in a process known as saponification. The fats are hydrolyzed by the base, yielding alkali salts of fatty acids (crude soap) and glycerol.

Today, soap is often replaced by other cleaning agents, such as synthetic detergents.