1. Music.
   1. Identity of pitch; the interval of a perfect prime.
   2. The combination of parts at the same pitch or in octaves.
2. The act or an instance of speaking the same words simultaneously by two or more speakers.
3. An instance of agreement; concord.

in unison

1. In complete agreement; harmonizing exactly.
2. At the same time; at once.

[Middle English, from Old French, from Medieval Latin ūnisonus, in unison, from Late Latin, monotonous : Latin ūni-, uni- + Latin sonus, sound.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.