The Collaborative International Dictionary of English v.0.48:

Interaction \In`ter*ac"tion\, n.
   1. Intermediate action.
      [1913 Webster]

   2. Mutual or reciprocal action or influence; as, the
      interaction of the heart and lungs on each other.
      [1913 Webster]

   3. Hence: (Physics) The effect, such as exertion of a force,
      that one object exerts on another, especially the capture
      or emission of a particle.
      [PJC]

   4. Communication between people, or the actions of people
      that affect others.
      [PJC]

WordNet (r) 3.0 (2006):

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: interaction,
       fundamental interaction]