The Collaborative International Dictionary of English v.0.48:

Dampen \Damp"en\ (d[a^]mp"'n), v. t. [imp. & p. p. dampened
   (d[a^]mp"'nd); p. pr. & vb. n. dampening.]
   1. To make damp or moist; to make slightly wet.
      [1913 Webster]

   2. To depress; to check; to make dull; to lessen.
      [1913 Webster]

            In a way that considerably dampened our enthusiasm.
                                                  --The Century.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

dampening \dampening\ n.
   the act or process of making something slightly wet.

   Syn: moistening.
        [WordNet 1.5]