What are pistachio nuts?

Pistachio nuts are the technically the edible seeds of the fruit from pistachio trees.  They are lower in calories than most other tree nuts.  One ounce or ~49 kernels provide ~170 calories.  They also are a good source of phytosterols, antioxidants, unsaturated fat, carotenoids, vitamins, minerals and fiber
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A member of the Anacardiaceae or cashew family, which also includes mangoes, the pistachio is the pea-sized seed of the small tree Pistacia vera. Its English name, pistachio, is derived from the Persian word for the tree,pistah, which in the conditions provided by Iran (modern-day Persia) - stony, poor soil, high heat, and little or no rainfall - can thrive for centuries, a fact attested to by a number of two-hundred-year-old trees still producing nuts in the Middle East and a seven-hundred-year-old tree still living in Iran.

The pistachio tree grows to a height of about 20 feet. Some trees are male, others female, and both are needed for reproduction, which, in this case, results in pistachio nuts. The male trees produce the pollen, which is carried on the wind to the female trees, on which the nuts, each enclosed in its own reddish green hull, grow in clusters like grapes. One male tree provides enough pollen for six female trees.

Pistachio trees mature slowly, do not begin to bear fruit until seven to ten years after planting, and take a full twenty years to reach peak production. Even then, like many other nut trees, the pistachio is alternate-year-bearing - producing a large crop one year and very little or nothing at all the next. The trees develop a brownish green flower in early summer, and the nuts ripen in late summer to early autumn.

Encased in a hard, pale beige shell that splits open partially when the bright green seed is mature, pistachios bless us with a rich nutty flavor underscored with a hint of sweetness. The visually appealing green nuts are quite popular in Mediterranean and Middle Eastern cuisines, where their color and flavor enhance both sweet and savory dishes. The introduction of the pistachio tree to California has resulted in a dramatic increase in their popularity in the United States as well. A favorite nut for eating out of hand, pistachios is also used in a wide variety of confections, baked goods, and desserts, in which their lively green hue adds to their appeal. Pistachios are also pressed for their delicious oil, and their pleasing color is used in green food coloring. The green in the pistachio nut is due to its content of chlorophyll, the pigment that makes plants green.