Is a peanut a “nut” or “legume”?

Both!

Botanically, peanuts are classified as legumes. Like other legumes, peanuts grow underground, as opposed to walnuts and almonds which grow on trees. These are botanically classified as drupes. However, for culinary, research and nutritional purposes, peanuts, like walnuts and almonds, are considered a nut. In fact, they provide the best source of plant-based protein when compared to other nuts, and their nutritional compounds have been shown to protect the body in various ways.

Where do they fit into the food Pyramid?

Today peanuts are classified in the USDA Food Guide Pyramid with meat, fish, poultry, dry beans, eggs and nuts because of their high protein content. However, because peanuts are plant-based, they do NOT contain cholesterol and some of the saturated fat that is found in animal-based sources of protein. In fact, they contain a much higher percentage of the "good" unsaturated fats than saturated fats. Therefore, peanuts have been placed closer to the bottom of both the Mediterranean and the Vegetarian Food Guide Pyramids, in the eat daily category.