The Collaborative International Dictionary of English v.0.48:

Equilibrium \E`qui*lib"ri*um\, n.; pl. E. Equilibriums, L.
   Equilibria. [L. aequilibrium, fr. aequilibris in
   equilibrium, level; aequus equal + libra balance. See
   Equal, and Librate.]
   1. Equality of weight or force; an equipoise or a state of
      rest produced by the mutual counteraction of two or more
      forces.
      [1913 Webster]

   2. A level position; a just poise or balance in respect to an
      object, so that it remains firm; equipoise; as, to
      preserve the equilibrium of the body.
      [1913 Webster]

            Health consists in the equilibrium between those two
            powers.                               --Arbuthnot.
      [1913 Webster]

   3. A balancing of the mind between motives or reasons, with
      consequent indecision and doubt.
      [1913 Webster]

   Equilibrium valve (Steam Engine), a balanced valve. See
      under Valve.
      [1913 Webster]

WordNet (r) 3.0 (2006):

equilibrium
    n 1: a stable situation in which forces cancel one another [ant:
         disequilibrium]
    2: a chemical reaction and its reverse proceed at equal rates
       [syn: chemical equilibrium, equilibrium]
    3: equality of distribution [syn: balance, equilibrium,
       equipoise, counterbalance]
    4: a sensory system located in structures of the inner ear that
       registers the orientation of the head [syn: equilibrium,
       labyrinthine sense, vestibular sense, sense of balance,
       sense of equilibrium]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "equilibrium":
   accordance, aplomb, assurance, balance, balanced personality,
   beauty, bilateral symmetry, calm, coequality, coextension,
   communion, community, concinnity, confidence, conformity,
   congruity, consistency, consonance, constancy, continuity, cool,
   corelation, correlation, correlativism, correlativity,
   correspondence, counterbalance, counterpoise, dynamic symmetry,
   equability, equality, equanimity, equation, equipoise,
   equipollence, equiponderance, equity, equivalence, equivalency,
   euphony, eurythmics, eurythmy, evenness, fastness, finish,
   firmness, harmony, homeostasis, homogeneity, identity,
   imperturbability, invariability, inverse proportion, inverse ratio,
   inverse relationship, justice, keeping, level head,
   levelheadedness, levelness, likeness, measure, measuredness,
   monolithism, multilateral symmetry, mutuality, nerve, order,
   orderedness, par, parallelism, parity, persistence, poise,
   polarity, possession, presence of mind, proportion,
   proportionality, reciprocality, reciprocation, reciprocity,
   regularity, relativity, reliability, restraint, rhythm, rootedness,
   sang-froid, secureness, security, self-assurance, self-command,
   self-confidence, self-control, self-possession, self-restraint,
   shapeliness, solidity, soundness, stability, stabilization,
   stable equilibrium, stable state, stasis, steadfastness,
   steadiness, steady nerves, steady state, substantiality, sweetness,
   symmetricalness, symmetry, trilateral symmetry, undeflectability,
   uniformity, unity, unruffledness, unshakable nerves,
   unshakableness, well-regulated mind