The Collaborative International Dictionary of English v.0.48:

Commemorative \Com*mem"o*ra*tive\, a.
   Tending or intended to commemorate; as, a commemorative
   plaque. "A sacrifice commemorative of Christ's offering up
   his body for us." --Hammond.
   [1913 Webster]

         An inscription commemorative of his victory. --Sir G.
                                                  C. Lewis.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Commemorative \Com*mem"o*ra*tive\, n.
   something that commemorates, especially a postage stamp or
   coin having a design commemorating some event, person,
   institution, etc.
   [PJC]

WordNet (r) 3.0 (2006):

commemorative
    adj 1: intended as a commemoration; "a commemorative plaque"
           [syn: commemorative, commemorating]
    n 1: an object (such as a coin or postage stamp) made to mark an
         event or honor a person