In music, syncopation is the stressing of normally unstressed beat in a bar. For example, in 4/4 time, the first and third beats are nomrally stressed; if instead the second and fourth beats are stressed and the first and third unstressed, the rhythm is syncopated.
The stress can also shift by less than a whole beat so it falls on an off-beat, as in the following example where the stress in the first bar is shifted by a quaver (or eighth-note):
Syncopation is used on occasion in many music styles, including classical music, but is is a fundamental constant presence in such styles as ragtime and jazz.