The Collaborative International Dictionary of English v.0.48:

Rachis \Ra"chis\ (r[=a]"k[i^]s), n.; pl. E. Rachises
   (r[=a]"k[i^]s*[e^]z), L. Rachides (r[a^]k"[i^]*d[=e]z).
   [NL., fr. Gr. "ra`chis, -ios.] [Written also rhachis.]
   1. (Anat.) The spine; the vertebral column.
      [1913 Webster]

   2. (Bot. & Zool.) Same as Rhachis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, rachis]