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]

WordNet (r) 3.0 (2006):

in unison
    adv 1: speaking or singing at the same time; simultaneously;
           "they shouted `Yes!' in unison"; "they responded in
           chorus to the teacher's questions" [syn: in unison, in
           chorus]
    2: at the same pitch; "they sang in unison"

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "in unison":
   accordant, according, agreeably, all agreeing, all together,
   as one, as one man, assonant, assonantal, at a clip, at once,
   at one time, attuned, back to back, blended, blending,
   by acclamation, by consensus, cheek by jowl, chiming, coactively,
   coefficiently, coinstantaneously, collectively, combinedly,
   communally, compatibly, concertedly, concordant, concordantly,
   concurrently, congenially, conjointly, consentaneously, consonant,
   consonantly, cooperatingly, cooperatively, corporately, fittingly,
   hand in glove, hand in hand, harmonic, harmonious, harmoniously,
   harmonizing, homophonic, in a chorus, in accord, in agreement,
   in chorus, in common, in concert, in concert with, in concord,
   in harmony with, in keeping, in line, in partnership, in phase,
   in step, in sync, in tune, inharmony, isochronously, jointly,
   just right, monodic, monophonic, mutually, nem con,
   nemine contradicente, nemine dissentiente, on all hands,
   on the beat, one and all, shoulder to shoulder, simultaneously,
   symphonious, synchronized, synchronous, synchronously, to a man,
   together, tuned, unanimously, unisonant, unisonous, with it,
   with one accord, with one consent, with one voice,
   without contradiction