Unison, n.
Etymology: [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.
1 more senses
3. A single, unvaried. [R.] Pope. In unison, in agreement; agreeing in tone; in concord.
1 other Webster entry under this spelling
Unison, a.
1. Sounding alone. [Obs.] [sounds] intermixed with voice, Choral or unison. Milton.
2. (Mus.)
Sounded alike in pitch; unisonant; unisonous; as, unison passages, in which two or more parts unite in coincident sound.
Shades of meaning
Fernald has no entry of its own for unison; he lists it under harmony, melody.
Where he writes about it:
Accordant notes of different pitch sounded simultaneously produce harmony; unison is the simultaneous sounding of two or more notes of the same pitch. — from melody
When the pitch is the same, there may be unison between sounds of very different volume and quality, as a voice and a bell may sound in unison. — from melody
Tones sounded at the interval of an octave are also said to be in unison, altho this is not literally exact; this usage arises from the fact that bass and tenor voices in attempting to sound the same note as the soprano and alto will in fact sound a note an octave below. — from melody