This word usually appears in the phrase "in unison", which means "together, at the same time" or "at the same musical pitch". So an excited crowd responding to a speaker may shout in unison, and a group of demonstrators may chant in unison. The old church music called Gregorian chant was written to be sung in unison, with no harmonizing voices, and kindergarten kids always sing in unison (at least when they can all find the same pitch). In a similar way, an aerobics class moves in unison following the instructor, and a group or even a whole town may work in unison when everyone agrees on a common goal.