NOUN (3)

1. corresponding exactly;
- Example: "marching in unison"

2. occurring together or simultaneously;
- Example: "the two spoke in unison"

3. (music) two or more sounds or tones at the same pitch or in octaves;
- Example: "singing in unison"

The Collaborative International Dictionary of English v.0.48:

Unison \U"ni*son\ (?; 277), n. [LL. unisonus having the same
   sound; L. unus one + sonus a sound: cf. F. unisson, It.
   unisono. See One, and Sound a noise.]
   1. Harmony; agreement; concord; union.
      [1913 Webster]

   2. (Mus.) Identity in pitch; coincidence of sounds proceeding
      from an equality in the number of vibrations made in a
      given time by two or more sonorous bodies. Parts played or
      sung in octaves are also said to be in unison, or in
      octaves.
      [1913 Webster]

   Note: If two cords of the same substance have equal length,
         thickness, and tension, they are said to be in unison,
         and their sounds will be in unison. Sounds of very
         different qualities and force may be in unison, as the
         sound of a bell may be in unison with a sound of a
         flute. Unison, then, consists in identity of pitch
         alone, irrespective of quality of sound, or timbre,
         whether of instruments or of human voices. A piece or
         passage is said to be sung or played in unison when all
         the voices or instruments perform the same part, in
         which sense unison is contradistinguished from harmony.
         [1913 Webster]

   3. A single, unvaried. [R.] --Pope.
      [1913 Webster]

   In unison, in agreement; agreeing in tone; in concord.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unison \U"ni*son\ (?; 277), a. [Cf. It. unisono. See Unison,
   n.]
   1. Sounding alone. [Obs.]
      [1913 Webster]

            [sounds] intermixed with voice,
            Choral or unison.                     --Milton.
      [1913 Webster]

   2. (Mus.) Sounded alike in pitch; unisonant; unisonous; as,
      unison passages, in which two or more parts unite in
      coincident sound.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unison
    n 1: corresponding exactly; "marching in unison"
    2: occurring together or simultaneously; "the two spoke in
       unison"
    3: (music) two or more sounds or tones at the same pitch or in
       octaves; "singing in unison"

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "unison":
   acclamation, accompaniment, accord, accordance, affinity, agape,
   agreement, agreement of all, amity, assent, attune, attunement,
   bonds of harmony, brotherly love, cantata, caritas,
   cement of friendship, charity, chime, chiming, choral singing,
   chorus, coetaneity, coetaneousness, coevalneity, coevalness,
   coexistence, coherence, coincidence, common assent, common consent,
   communion, community, community of interests, compatibility,
   concentus, concert, concomitance, concord, concordance,
   concurrence, conformance, conformation, conformity, congeniality,
   congruence, congruency, congruity, consensus, consensus gentium,
   consensus of opinion, consensus omnium, consent, consentaneity,
   consistency, consonance, consonancy, consonant, consort,
   contemporaneity, contemporaneousness, cooperation, correspondence,
   diapason, empathy, equivalence, esprit, esprit de corps, euphony,
   feeling of identity, fellow feeling, fellowship, frictionlessness,
   general acclamation, general agreement, general consent,
   general voice, glee, good vibes, good vibrations, happy family,
   harmonics, harmonious, harmony, heavy harmony, homophony, identity,
   in harmony, intersection, isochronism, kinship, like-mindedness,
   love, madrigal, madrigaletto, meeting of minds, monochord, monody,
   mutual understanding, mutuality, one accord, one voice, oneness,
   oratorio, overlap, parallelism, peace, rapport, rapprochement,
   reciprocity, same mind, self-consistency, sharing, simultaneity,
   single voice, solidarity, symmetry, sympathy, symphony, sync,
   synchronism, synchronization, tally, team spirit,
   three-part harmony, timing, together, total agreement, tune,
   unanimity, unanimousness, understanding, uniformity, union,
   unisonance, unity, universal agreement