Achenes are a type of fruits where the fruit is dry and just barely bigger than the seed. As they don't have any juice, they are never eaten as a fruit culinarily.
They are very, very similar to cypselae, as they are both dry, indehiscent fruits with a single seed linked to the fruit wall only by the funicle. However achenes are simple fruits and derive from the superior ovaries. Cypselae instead originate from inferior ovaries.