Nut (fruit)

A nut is a type of fruit with a hard, tough shell that protects a single seed inside. Botanically, the term "nut" specifically refers to fruits whose shells do not naturally open to release the seed—a characteristic called indehiscence. Unlike many other seeds, which are freed from their fruit on their own, true nuts like hazelnuts, chestnuts, and acorns have a hard shell wall and come from a compound ovary. Because they stay closed at maturity, these nuts depend on outside help for seed dispersal, often through animals that eat them and later excrete the seeds.

The seed itself is a mature, fertilized ovule made up of three parts: an embryo that can grow into a new plant, stored food for the embryo, and a protective seed coat. Botanically, a nut is a fruit with a woody outer layer (pericarp) that develops from a syncarpous gynoecium. Some nuts are surrounded by an involucre—a cup-shaped structure formed from flower bracts—which can be scaly, spiny, leafy, or tubular depending on the species. Most nuts come from flowers with inferior ovaries, and all are indehiscent. True nuts are produced by plant families in the order Fagales, including beech, chestnut, oak, stone-oak, and tanoak (family Fagaceae), as well as hazel, filbert, and hornbeam (family Betulaceae).

A small nut is sometimes called a "nutlet" (formerly a nucule, though that term now usually refers to the oogonium of stoneworts). In botany, "nutlet" can also describe a pyrena or pyrene—a seed covered by a stony layer, like the kernel inside a drupe. Walnuts and hickories (including pecans) from the family Juglandaceae have fruits that are hard to classify. Some definitions, including that of the authoritative Angiosperm Phylogeny Group, consider them nuts, while others call them drupaceous nuts.

Many nuts used for food can cause allergic reactions, ranging from mild symptoms to severe, life-threatening anaphylaxis. This reaction happens when the body releases histamine in response to an allergen in the nuts, affecting the skin and other systems. Tree nut allergies are different from peanut allergies—peanuts are legumes, while tree nuts are hard-shelled nuts—but experts advise that people allergic to peanuts should also avoid tree nuts, and vice versa.

For both wildlife and humans, nuts provide a high number of calories, mainly from unsaturated fats like linoleic and linolenic acids, as well as monounsaturated fats. They are rich in B vitamins, vitamin E, and essential amino acids. While nutrient content varies among different nuts, they generally have low water and carbohydrate levels and are high in protein, dietary minerals, and dietary fiber. Many nuts and seeds are edible and used in cooking: they can be eaten raw, sprouted, or roasted as a snack; soaked and filtered to make nut milk; ground into nut butters; or pressed for oil used in cooking and cosmetics.