Flying in air fresh raw whole and cracked pistachios  isolated on white background. High resolution image
The Scientific Reason Pistachios Aren't Technically Nuts
By Haldan Kirsch
Pistachios have been around for a very long time, as studies show that even early neanderthals enjoyed this green nut. However, while pistachios are most commonly referred to as nuts, they belong to a whole other food group which brings them closer in line with mangoes and peaches.
Pistachios are actually something called a drupe, which is a fleshy fruit that usually contains a single seed like peaches, plums, and cherries, or comes in small clumps like raspberries and blackberries. The reason pistachios don’t resemble any of these drupes is because of the way we’re exposed to them. 
Pistachios are actually just the inner seed of a pink and yellow, fleshy tree fruit that is discarded when it's harvested, and its delicious inner seed is roasted and sold inside its remaining shell instead. These types of nuts are not considered traditional nuts, but they are instead called ‘culinary nuts.’
Culinary nuts refer to the many drupes and legumes that are commonly known as nuts. So, while pistachios, walnuts, pecans, and cashews are technically the seeds of drupes, we use them in recipes as if they were nuts.