A small, dry, indehiscent one-seeded fruit with a thin wall, as in a sunflower.
Any small, dry, indehiscent fruit, as the strawberry, with one seed which is attached to the ovary wall at only one point.
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.