The Collaborative International Dictionary of English v.0.48:

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, such as one of an iris. See Rootstock.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rhizome
    n 1: a horizontal plant stem with shoots above and roots below
         serving as a reproductive structure [syn: rhizome,
         rootstock, rootstalk]