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]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "interaction":
   ESP, alternation, answer, coaction, commerce, communication,
   communion, complementary distribution, concurrence, congress,
   connection, contact, conversation, converse, cooperation,
   correspondence, dealing, dealings, engagement, exchange,
   information, interchange, intercommunication, intercommunion,
   intercourse, interlacing, intermeshing, interplay, intertwining,
   interweaving, interworking, linguistic intercourse, mesh, meshing,
   message, quid pro quo, reply, response, seesaw, social intercourse,
   speaking, speech, speech circuit, speech situation, talking,
   telepathy, tit for tat, touch, traffic, truck,
   two-way communication