Beat (music)
So "off-beat" is a musical term in syncopation that emphasizes the weak even beats of a bar, as opposed to the usual on-beat. This is a basic beat in African polyrhythm that was transferred over into popular western music. According to Grove Music, the "Offbeat is [often] where the downbeat is replaced by a rest or is carried over from the preceding bar". The downbeat (the first beat of a measure in music) can never be the off-beat because it is the strongest beat in 4/4 time. Certain genres tend to emphasize the off-beat; especially rock and roll and Ska music.