An achene is a type of simple dry fruit produced by many species of flowering plants. Achenes are monocarpellate (formed from one carpel) and indehiscent (do not open at maturity) and contain a single seed that nearly fills the pericarp (fruit layers), but does not adhere to it. In many species, what we think of as the "seed" is actually an achene, a fruit containing the seed. Typical achenes are the fruits of buttercup and buckwheat. It is sometimes spelled akene, and rarely called achenium or achenocarp.
Other fruits, such as milkweed and strawberry, are sometimes considered to be types of achenes, although these are accessory fruits. Some of these are bicarpellate. In the case of the strawberry, the fruits (achenes) are the tiny "seed-like" structures on the outside of the fleshy part, which is a modified stem or torus.
Fruits of sedges are also considered achenes sometimes because it has a one-locule compound ovary. By the same definition, the common fruit type in the Family Asteraceae is also usually considered achene (some term the asteraceous achene cypsela, however). A sunflower "seed" in the husk is not really a seed, but an achene. The white-gray husks are the walls of the fruit.
A grain, a type of fruit closely resembling an achene, differs in that the pericarp is fused to the thin seed coat in the grain.
A winged achene, such as in maple, is samara.
Utricle is like an achene, but it doesn't have a simple ovary, but a compound one. In addition, its fruit ovary becomes bladdery or corky.