nut

nut, in botany, dry hard fruit that does not split open at maturity to release its single seed. A nut resembles an achene but develops from more than one carpel (female reproductive structure), often is larger, and has a tough woody wall. Examples of true nuts are the chestnut, hazelnut, and acorn.

Many edible oily seeds are popularly called “nuts,” especially those with a hard shell. Many of these culinary nuts are the seeds of drupe fruits, including walnuts, pistachios, almonds, and coconuts. The peanut is a legume, and the Brazil nut is a seed from a capsule fruit.