(n.) The quality or condition of being reciprocal; reciprocalness.

Lissajous figure back-and-forth battledore and shuttlecock beat chain coaction coming and going commutation complementary distribution concurrence consecution cooperation counterchange cross fire cyclicalness ebb and flow engagement exchange fluctuation flux and reflux give-and-take interaction interchange intercommunication intercourse interlacing intermeshing intermittence intermittency intermutation interplay intertwining interweaving interworking lex talionis measure for measure mesh meshing meter mutual admiration mutual support mutual transfer mutuality order oscillation pendulation pendulum motion periodicalness periodicity permutation piston motion progression pulsation quid pro quo reappearance reciprocality reciprocation reciprocity recurrence regular wave motion reoccurrence repetition retaliation return reversion rhythm rhythmic play rotation row seasonality seesaw seesawing sequel sequence series shifting shuffling sine wave something for something succession teeter teeter-totter teeter-tottering teetering tit for tat to-and-fro tottering train transposal transposition undulation ups and downs vacillation variation vicissitude wavering wigwag