The Collaborative International Dictionary of English v.0.48:

Centerpiece \Cen"ter*piece`\, Centrepiece \Cen"tre*piece`\
   (s[e^]n"t[~e]r*p[=e]s`), n.
   1. An ornament to be placed in the center, as of a table,
      ceiling, atc.; a central article or figure.
      [1913 Webster]

   2. Specifically: An arrangement of flowers placed in the
      center of a table as an ornament while dining.
      [PJC]

   3. The central or most important feature of an activity,
      plan, or object; as, the centerpiece of her campaign was a
      proposal to slash taxes.
      [PJC] Center punch

WordNet (r) 3.0 (2006):

centerpiece
    n 1: the central or most important feature; "education was the
         centerpiece of the Democratic Party's political platform"
         [syn: centerpiece, centrepiece]
    2: something placed at the center of something else (as on a
       table) [syn: centerpiece, centrepiece]