A neutral salt made by reacting a fatty acid with a caustic alkali, and used in washing and cleaning. One end of a soap molecule is attracted to water; the other to nonpolar substances such as oil. Thus, when mixed with water, soap helps dissolve ordinarily-insoluble oil and oily dirt. The term "soap" is also used casually to refer to detergents, a class of synthetic chemicals which work similarly but are more chemically stable than true soaps.

Soaps and detergents are not exactly interchangeable, even though they do roughly the same things with oil and water. Detergents, being more stable than soaps, are less affected by hard water -- but are also frequently less biodegradable. Early detergents were about as biodegradable as plastic, and caused serious pollution problems; luckily, they've gotten better since then.

Modern soaps, such as Dr. Bronner's, are made from vegetable oils and potassium hydroxide -- not animal fat and lye.

Soap is a surfactant, meaning that it reduces the surface tension of water. This has the amusing effect of helping water form bubbles.