The Collaborative International Dictionary of English v.0.48:

Equipoise \E"qui*poise\, n. [Equi- + poise.]
   1. Equality of weight or force; hence, equilibrium; a state
      in which the two ends or sides of a thing are balanced,
      and hence equal; state of being equally balanced; -- said
      of moral, political, or social interests or forces.
      [1913 Webster]

            The means of preserving the equipoise and the
            tranquillity of the commonwealth.     --Burke.
      [1913 Webster]

            Our little lives are kept in equipoise
            By opposite attractions and desires.  --Longfellow.
      [1913 Webster]

   2. Counterpoise.
      [1913 Webster]

            The equipoise to the clergy being removed. --Buckle.
      Equipollence

WordNet (r) 3.0 (2006):

equipoise
    n 1: equality of distribution [syn: balance, equilibrium,
         equipoise, counterbalance]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "equipoise":
   balance, ballast, coequality, coextension, consideration,
   correspondence, counterbalance, counterpoise, counterweight,
   equality, equation, equilibrium, equipollence, equiponderance,
   equity, equivalence, equivalency, equivalent, evenness,
   give-and-take, identity, justice, levelness, likeness, makeweight,
   offset, par, parallelism, parity, poise, proportion, quid pro quo,
   setoff, something of value, stasis, symmetry, tit for tat