The Collaborative International Dictionary of English v.0.48:

Interplay \In`ter*play`\, n.
   Mutual action or influence; interaction; as, the interplay of
   affection.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interplay
    n 1: reciprocal action and reaction

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "interplay":
   ESP, alternation, answer, battledore and shuttlecock, coact,
   coaction, commerce, communication, communion, commutation,
   complementary distribution, concurrence, congress, connection,
   contact, conversation, converse, cooperate, cooperation,
   correspondence, counterchange, cross fire, dealing, dealings,
   dovetail, engage, engagement, exchange, give-and-take, information,
   interact, interaction, interchange, intercommunication,
   intercommunion, intercourse, interlace, interlacing, intermesh,
   intermeshing, intermutation, intertwine, intertwining, interweave,
   interweaving, interwork, interworking, lex talionis,
   linguistic intercourse, measure for measure, mesh, meshing,
   message, mortise, mutual admiration, mutual support,
   mutual transfer, mutuality, permutation, quid pro quo,
   reciprocality, reciprocation, reciprocity, reply, response,
   retaliation, seesaw, social intercourse, something for something,
   speaking, speech, speech circuit, speech situation, talking,
   telepathy, tit for tat, touch, traffic, transposal, transposition,
   truck, two-way communication