Syncopation
In music, syncopation is a stress on a normally unstressed beat, or a missing beat where a stressed one would normally be expected. Syncopation is used in many musical styles, including classical music, but it is fundamental in such styles as reggae, ragtime, rap, jump blues, jazz and often in dubstep. In the form of a back beat, syncopation is used in virtually all contemporary popular music.
The link address is: http://en.wikipedia.org/wiki/Syncopation