Source : Webster's Revised Unabridged Dictionary (1913)

Rachis \Ra"chis\, n.; pl. E. {Rachises}, L. {Rachides}. [NL.,
   fr. Gr. ? ?.] [Written also {rhachis}.]
   1. (Anat.) The spine; the vertebral column.

   2. (Bot. & Zo["o]l.) Same as {Rhachis}.

Source : WordNet®

rachis
     n 1: axis of a compound leaf or compound inflorescence
     2: the series of vertebrae forming the axis of the skeleton and
        protecting the spinal cord; "the fall broke his back"
        [syn: {spinal column}, {vertebral column}, {spine}, {backbone},
         {back}]
     [also: {rhachises} (pl)]