The Collaborative International Dictionary of English v.0.48:

Equivalence \E*quiv"a*lence\, v. t.
   To be equivalent or equal to; to counterbalance. [R.] --Sir
   T. Browne.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Equivalence \E*quiv"a*lence\ ([-e]*kw[i^]v"[.a]*lens), n. [Cf.
   F. ['e]quivalence, LL. aequivalentia.]
   1. The condition of being equivalent or equal; equality of
      worth, value, signification, or force; as, an equivalence
      of definitions.
      [1913 Webster]

   2. Equal power or force; equivalent amount.
      [1913 Webster]

   3. (Chem.)
      (a) The quantity of the combining power of an atom,
          expressed in hydrogen units; the number of hydrogen
          atoms can combine with, or be exchanged for; valency.
          See Valence.
      (b) The degree of combining power as determined by
          relative weight. See Equivalent, n., 2. [R.]
          [1913 Webster]

WordNet (r) 3.0 (2006):

equivalence
    n 1: a state of being essentially equal or equivalent; equally
         balanced; "on a par with the best" [syn: equality,
         equivalence, equation, par]
    2: essential equality and interchangeability [ant:
       nonequivalence]
    3: qualities that are comparable; "no comparison between the two
       books"; "beyond compare" [syn: comparison, compare,
       equivalence, comparability]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "equivalence":
   accord, accordance, affinity, agreement, analogousness, assent,
   balance, chorus, coequality, coextension, coherence, coincidence,
   commensurability, communion, community, comparability,
   comparableness, comparativeness, compatibility, concert, concord,
   concordance, conformance, conformation, conformity, congeniality,
   congruence, congruency, congruity, consistency, consonance,
   consort, cooperation, corelation, correlation, correlativism,
   correlativity, correspondence, equality, equation, equilibrium,
   equipoise, equipollence, equiponderance, equity, equivalency,
   evenness, harmony, homogeneity, homoousia, identity,
   indistinguishability, intersection, inverse proportion,
   inverse ratio, inverse relationship, justice, levelness, likeness,
   mutuality, no difference, oneness, overlap, par, parallelism,
   parity, peace, poise, proportion, proportionability,
   proportionality, rapport, ratio, reciprocality, reciprocation,
   reciprocity, relativity, sameness, self-consistency, self-identity,
   selfhood, selfness, selfsameness, similarity, symmetry, sync,
   synchronism, synonymity, synonymousness, synonymy, tally, timing,
   uniformity, union, unison, unisonance, unity