The botanical definition of a “nut” is a fruit whose ovary wall becomes very hard at maturity. Using this criterion, the peanut is not a nut but rather a legume. As a legume the peanut is commonly known as the bean or pea family. Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in root nodules. This capacity to fix nitrogen means peanuts require less nitrogen-containing fertilizer and improve soil fertility, making them valuable in crop rotations.

source: internet