- any small, dry, hard, one-seeded, indehiscent fruit.
- a dry one-seeded indehiscent fruit with the seed distinct from the fruit wall. It may be smooth, as in the buttercup, or feathery, as in clematis
- A small, dry, one-seeded fruit in which the seed sits free inside the hollow fruit, attached only by the stem of the ovule. Achenes are indehiscent (they do not split open when ripe). The fruits of the sunflower and elm are achenes.