1
general::
1. In complete agreement, harmonizing exactly. For example, Their opinion was in unison with ours. [Early 1800s] 2. Saying the same thing at the same time, simultaneously, as in The whole class answered in unison. [Late 1800s] Both usages allude to the unison of music, a single identical pitch.
American Heritage Idioms
2
general::
Phrase(s): in unison
[of musical notes, instruments, or voices] having the same pitch. • This part of the piece is performed in unison. • The twins sang in unison.
McGrawhill's American Idioms And Phrasal Verbs