The Collaborative International Dictionary of English v.0.48:

Vertebra \Ver"te*bra\ (v[~e]r"t[-e]*br[.a]), n.; pl.
   Vertebrae. [L. vertebra, fr. vertere to turn, change. See
   Verse.]
   [1913 Webster]
   1. (Anat.) One of the serial segments of the spinal column.
      [1913 Webster]

   Note: In many fishes the vertebrae are simple cartilaginous
         disks or short cylinders, but in the higher vertebrates
         they are composed of many parts, and the vertebrae in
         different portions of the same column vary very
         greatly. A well-developed vertebra usually consists of
         a more or less cylindrical and solid body, or centrum,
         which is surmounted dorsally by an arch, leaving an
         opening which forms a part of the canal containing the
         spinal cord. From this dorsal, or neural, arch spring
         various processes, or apophyses, which have received
         special names: a dorsal, or neural, spine, spinous
         process, or neurapophysis, on the middle of the arch;
         two anterior and two posterior articular processes, or
         zygapophyses; and one or two transverse processes on
         each side. In those vertebrae which bear well-developed
         ribs, a tubercle near the end of the rib articulates at
         a tubercular facet on the transverse process
         (diapophysis), while the end, or head, of the rib
         articulates at a more ventral capitular facet which is
         sometimes developed into a second, or ventral,
         transverse process (parapophysis). In vertebrates with
         well-developed hind limbs, the spinal column is divided
         into five regions in each of which the vertebrae are
         specially designated: those vertebrae in front of, or
         anterior to, the first vertebra which bears ribs
         connected with the sternum are cervical; all those
         which bear ribs and are back of the cervicals are
         dorsal; the one or more directly supporting the pelvis
         are sacral and form the sacrum; those between the
         sacral and dorsal are lumbar; and all those back of the
         sacral are caudal, or coccygeal. In man there are seven
         cervical vertebrae, twelve dorsal, five lumbar, five
         sacral, and usually four, but sometimes five and rarely
         three, coccygeal.
         [1913 Webster]

   2. (Zool.) One of the central ossicles in each joint of the
      arms of an ophiuran.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

vertebra
    n 1: one of the bony segments of the spinal column