Noah Webster's New International Dictionary of the English Language

1. (n.) Harmony; agreement; concord; union.

2. (n.) 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.

3. (n.) A single, unvaried.

4. (n.) Sounding alone.

5. (n.) Sounded alike in pitch; unisonant; unisonous; as, unison passages, in which two or more parts unite in coincident sound.