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]