Castor oil is a vegetable oil obtained from the castor bean (or preferably castor seed as the castor plant (Ricinus communis L.) is not a member of the bean family).

Castor oil is extremely versatile and unique in its composition. Castor oil consists of 90% unsaturated C:18 ricinoleic fatty acid. It is a major source of sebacic acid.

Castor oil and its derivatives have major applications in the manufacturing of soaps, lubricants, hydraulic and brake fluids, paints, dyes, coatings , inks, cold resistant plastics, waxes and polishes, nylon, pharmaceuticals and perfumes.

The poison ricin is made from the byproducts in the manufacture of castor oil.

About 1% of the global castor oil production goes into medical or health store products. It is used to ease constipation and as an emetic to induce vomiting. Consumption of large amounts of castor oil (below lethal doses, such as one bottle) can induce labor in near-term pregnant women. It is notorious for its strong taste, which was for many years a standard gag in comic strips and Our Gang film shorts .

Castor oil as a tool of political terror

In Fascist Italy under the regime of Benito Mussolini, castor oil was one of the tools of the blackshirts. Political dissidents would be force-fed large dosages of castor oil by Fascist paramilitary groups. This technique was said to have been originated by Gabriele D'Annunzio. Victims of this treatment would experience severe diarrhea and dehydration, often resulting in death.

Sometimes when the blackshirts wished to make sure that the victim would die rather than simply be badly disabled, they would mix gasoline with the castor oil.

It came to be said that Mussolini's power was backed by "the bludgeon and castor oil".