Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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.

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

   {In unison}, in agreement; agreeing in tone; in concord.

Source : WordNet®

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 chorus}]
     2: in complete agreement; "my feelings about the matter are in
        unison with yours" [syn: {in accord}]
     3: at the same pitch; "they sang in unison"