An interval before the beginning of a piece of music during which the conductor (bandleader, click track) establishes the time signature and tempo of the music to be played. In most Western popular music, a count off is considered essential to ensuring uniform entrances from the instruments and voices in the ensemble.
Count offs are usually one or two measures in length and are normally vocalized in 4/4 meter as, “One, Two, Three, Four” or the metrically incorrect but common, “One – Two – One, Two, Three, Four”. (The initial “two” actually lands on beat three of the first measure.) In the studio, the final beat in the countoff is often silent, so that there it doesn’t bleed into open microphones as the track begins: “One, Two, Three, Four, One, Two, Three, (silent count)”.
Ensemble music doesn’t always require a count off. Orchestral musicians are trained to respond to the conductor’s downbeat and begin playing on (actually slightly after) beat one. Even classical ensembles without conductors rely on a simple downbeat, usually provided by the principal violinist – or, as in Mozart’s day, the keyboardist.