Hazelnuts from the Common Hazel

A nut in botany is a one-seeded (rarely two) simple dry fruit in which the ovary wall or part of it becomes very hard (stony or woody) at maturity. Most nuts come from pistils with inferior ovaries (see flower) and all are indehiscent (do not open at maturity). Examples of true nuts are the fruits produced by oak (acorn), hazelnut (filbert), beech, and butternut trees.

"Nuts" in cuisine are a much less restrictive category than nuts in botany, the term being applied (or misapplied, depending upon your orientation) to many seeds that are not true nuts. Any large, oily kernel found within a shell and used in food may be regarded as a nut. Because nuts generally have a high oil content, they are a highly prized food and energy source. A large number of seeds are edible by humans and used in cooking, eaten raw, sprouted, or roasted as a snack food, or pressed for oil that is used in cookery and cosmetics. By the same token, nuts (or seeds generally) are a significant source of nutrition for wildlife. This is particularly true in temperate climates where animals such as jays and squirrels store acorns and other nuts during the autumn to keep them from starving during the winter and early spring.

Nuts of temperate climates are dominated by wind-pollinated trees of the Order Fagales:

• Acorn, the seed of the oak tree
• Beech
• Hazelnut or filbert, the seed of the hazel and cob trees
• Chestnut

Other members of Fagales with nut-like fruits include walnut, pecan and hickory

Some fruits and seeds that are "nuts" in the culinary sense but not in the botanical sense:

• Almond, walnut, pecan and hickory are the edible seeds of drupes - the leathery "flesh" is removed at harvest.
• Brazil nuts are seeds from a capsule.
• Cashew nuts are seeds.
• Coconut is a dry fiberous drupe.
• horse-chestnut (not edible!) is a capsule.
• Peanut is a legume.
• Pine nut or pinyon is the seed of several species of pine (coniferous trees).
• Candle nut (used for oil) is a seed.
• Pistachio nut is the seed of a thin-shelled drupe.

The "nut" of the horse-chestnut, (Aesculus hippocastanum), is also known as a conker. Conkers are inedible but are collected and used in an old children's game, also known as conkers, in which a nut is threaded onto a strong cord and then each child attempts to break their opponent's conker by hitting it with their own. A related species, Aesculus californica, was formerly eaten by the Native Americans of California in times of famine. It must be leached to remove poisonous constituents before eating.