The Collaborative International Dictionary of English v.0.48:

carcinogen \car*cin"o*gen\ (k[aum]r*s[i^]n"[-o]*j[e^]n), n.
   Any substance that produces cancer; as, to test for
   carcinogens in the drinking water.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

carcinogen
    n 1: any substance that produces cancer