They are often referred to as "tree nuts," but they are actually seeds that grow on the cashew tree (Anacardium occidentale). They actually belong to the same family as mangoes and pistachios.

The cashew tree (Anacardium occidentale) is a tropical evergreen tree that produces the cashew seed and the cashew apple accessory fruit. The tree can grow as high as 14 m (46 ft), but the dwarf ...

The trees are a member of the same family as poison ivy, and like poison ivy, cashew plants contain a toxin called urushiol. It's found in the plant itself and the brown oil inside a nut's shell.