Source : Webster's Revised Unabridged Dictionary (1913)

Rhachis \Rha"chis\, n.; pl. E. {Rhachises}, L. {Rhachides}. [See
   {Rachis}.] [Written also {rechis}.]
   1. (Anat.) The spine.

   2. (Bot.)
      (a) The continued stem or midrib of a pinnately compound
          leaf, as in a rose leaf or a fern.
      (b) The principal axis in a raceme, spike, panicle, or
          corymb.

   3. (Zo["o]l.)
      (a) The shaft of a feather. The rhachis of the
          after-shaft, or plumule, is called the hyporhachis.
      (b) The central cord in the stem of a crinoid.
      (c) The median part of the radula of a mollusk.
      (d) A central cord of the ovary of nematodes.

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)]

rhachises
     See {rachis}