Source : Webster's Revised Unabridged Dictionary (1913)

Rhizome \Rhi*zome"\, n. [Gr. ??? the mass of roots (of a tree),
   a stem, race, fr. ??? to make to root, pass., to take root,
   fr. ??? a root: cf. F. rhizome.] (Bot.)
   A rootstock. See {Rootstock}.

Source : WordNet®

rhizome
     n : a horizontal plant stem with shoots above and roots below
         serving as a reproductive structure [syn: {rootstock}, {rootstalk}]