Soap is a surfactant used in conjunction with water for washing and cleaning. Soap making usually comes in a solid moulded form, termed bars due to its historic and most typical shape. In this century, the use of thick liquid soap has become increasingly widespread, especially from soap dispensers in public washrooms. Applied to a soiled surface, soapy water effectively holds particles in suspension so the whole of it can be rinsed off with clean water. In the developed world, synthetic detergents have superseded soap as a laundry aid.

Many soaps are mixtures of sodium (soda) or potassium (potash) salts of fatty acids which can be derived from oils or fats by reacting them with an alkali (such as sodium or potassium hydroxide) at 80??100 ?C in a process known as saponification. The fats are hydrolyzed by the base, yielding glycerol and crude soap. Historically, the alkali used was potassium made from the deliberate burning of vegetation such as bracken, or from wood ashes.

Soap is derived from either oils or fats. Sodium Tallowate, a common ingredient in many soaps, is in fact derived from rendered beef fat. Soap can also be made of vegetable oils, such as olive oil. Soap made entirely from such oils, or nearly so, is called castile soap. The use of the word "soap" has become such a household name that even cleaning solutions for the body that don't have Soap making in the ingredients are referred to as soap.