Cashew oil refers to oil extracted from the fruit of the cashew

Cashew

The cashew is a tree in the family Anacardiaceae. Its English name derives from the Portuguese name for the fruit of the cashew tree, caju, which in turn derives from the indigenous Tupi name, acajú. It is now widely grown in tropical climates for its cashew nuts and cashew apples.-Etymology:The...

. The cashew nut has a shell with an oily outer layer, and a hard inner layer, within which the familiar cashew nut is contained.

Cashew shell oil

Cashew shell oil comes from the outer shell. It is toxic to humans, and is normally considered a waste product. Some work, however, showed that cashew shell oil can be useful in fighting dental cavities.
Cashew oil is used to treat wood and is believed to prevent termite attack
Other uses of cashew shell oil have been explored, including as an additive to brake fluid

Brake fluid

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure...

, to reduce brake fade and brake lining wear.
Composite Technical Services (Kettering, Ohio, USA) have researched the use of cashew shell oil as a resin for carbon composite products.