The Collaborative International Dictionary of English v.0.48:

Centennial State \Centennial State\
   Colorado; -- a nickname alluding to the fact that it was
   admitted to the Union in the centennial year, 1876.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Centennial State
    n 1: a state in west central United States in the Rocky
         Mountains [syn: Colorado, Centennial State, CO]