Syncopation means unexpected rhythms, which use the off-beat. It is created by emphasising the weak instead of the strong beats of the bar, such as by putting rests or holding notes over on strong beats.
Although syncopation has been used since medieval times, it was jazz musicians in the twentieth century who took syncopation to a whole new level, often through improvisation. Syncopation is also an important part of modern dance, reggae, pop and rock music.