Lissajous figure
alternation
back-and-forth
balance
battledore and shuttlecock
boomerang
coequality
comeback
coming and going
communion
community
commutation
cooperation
corelation
correlation
correlativism
correlativity
correspondence
counter
counterblast
counterblow
counterchange
counterstroke
cross fire
ebb and flow
equilibrium
equipollence
equivalence
exchange
flux and reflux
give-and-take
interchange
intermutation
interplay
inverse proportion
inverse ratio
inverse relationship
lex talionis
measure for measure
mutual admiration
mutual support
mutual transfer
mutuality
permutation
proportionality
quid pro quo
reciprocality
reciprocity
recoil
relativity
getting even
motility
motion
move
movement
paying back
return
Judges
accumulate
aesthetic
amity
amphoteric
freeze-out
intercommunication
like
newsboy
non-action
otherness
paralogism
polarisation
pubmed
r-squared
re-union
untranslated
upbraiding