The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]