Unison is complete agreement, or everybody sounding the same note at the same time. One voice, from many.
Music has the precise sense, since singing in unison means singing the same notes rather than in harmony, and it is a genuinely different effect. The everyday sense is about coordination, so acting in unison and nodding in unison both mean everybody doing the same thing together. The word almost always appears as in unison. It comes from Latin parts meaning one sound.
1noununisonSimultaneous performance of the same notes or actions.
“The whole room answered in unison.”
Medieval Latin
ūnisonus — having the same sound
Middle French
unisson
Middle English
unisoun
English, today
unison
From Middle English unisoun, from Middle French unisson, from Medieval Latin ūnisonus (“having the same sound”), from ūni- + sonus.
Etymology from Wiktionary, CC BY-SA