The Collaborative International Dictionary of English v.0.48:

Contamination \Con*tam`i*na"tion\
   (k[o^]n*t[a^]m`[i^]*n[=a]"sh[u^]n), n. [L. contaminatio.]
   The act or process of contaminating; pollution; defilement;
   taint; also, that which contaminates.
   [1913 Webster]

WordNet (r) 3.0 (2006):

contamination
    n 1: the state of being contaminated [syn: contamination,
         taint]
    2: a substance that contaminates [syn: contaminant,
       contamination]
    3: the act of contaminating or polluting; including (either
       intentionally or accidentally) unwanted substances or factors
       [syn: contamination, pollution] [ant: decontamination]