The Collaborative International Dictionary of English v.0.48:

Tuatara \Tu`a*ta"ra\, n. [Maori tuat[`a]ra; tua on the farther
   side (the back) + tara spine.] (Zool.)
   A large iguanalike reptile (Sphenodon punctatum) formerly
   common in New Zealand, but by 1900 confined to certain islets
   near the coast. It reaches a length of two and a half feet,
   is dark olive-green with small white or yellowish specks on
   the sides, and has yellow spines along the back, except on
   the neck. It is the only surviving member of the order
   Rhyncocephala. Also called tuatera and hatteria.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

Sphenodon punctatum
    n 1: only extant member of the order Rhynchocephalia of large
         spiny lizard-like diapsid reptiles of coastal islands off
         New Zealand [syn: tuatara, Sphenodon punctatum]