unison

Short definition from Wiktionary

Noun

1. 1
   Identical pitch between two notes or sounds; the simultaneous playing of notes of identical pitch (or separated by one or more octaves).
2. 2
   A sound or note having the same pitch as another, especially when used as the base note for an interval; a unison string.
3. 3
   The state of being in harmony or agreement; harmonious agreement or togetherness, synchronisation.
4. 4
   Two or more voices speaking or singing the same words together.

Proper noun

1. 1
   A public-sector trade union in the United Kingdom.