The Collaborative International Dictionary of English v.0.48:

Notochord \No"to*chord\, n. [Gr. nw^ton the back + E. chord.]
   (Anat.)
   An elastic cartilagelike rod which is developed beneath the
   medullary groove in the vertebrate embryo, and constitutes
   the primitive axial skeleton around which the centra of the
   vertebrae and the posterior part of the base of the skull are
   developed; the chorda dorsalis. See Illust. of Ectoderm.
   [1913 Webster]

WordNet (r) 3.0 (2006):

notochord
    n 1: a flexible rodlike structure that forms the supporting axis
         of the body in the lowest chordates and lowest vertebrates
         and in embryos of higher vertebrates