Source : Webster's Revised Unabridged Dictionary (1913)

Interaction \In`ter*ac"tion\, n.
   1. Intermediate action.

   2. Mutual or reciprocal action or influence; as, the
      interaction of the heart and lungs on each other.

Source : WordNet®

interaction
     n 1: a mutual or reciprocal action; interacting
     2: (physics) the transfer of energy between elementary
        particles or between an elementary particle and a field or
        between fields; mediated by gauge bosons [syn: {fundamental
        interaction}]