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From 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]