Source : Webster's Revised Unabridged Dictionary (1913)

Achene \A*chene"\, Achenium \A*che"ni*um\n. [Gr. 'a priv. + ? to
   gape.] (Bot.)
   A small, dry, indehiscent fruit, containing a single seed, as
   in the buttercup; -- called a naked seed by the earlier
   botanists. [Written also {akene} and {ach[ae]nium}.]

Source : WordNet®

achene
     n : small dry indehiscent fruit with the seed distinct from the
         fruit wall