Syncopation

Syncopation is music which creates rhythmical variation. Examples would be:
"If a part of the measure (bar) that is usually unstressed is accented, the rhythm is considered to be syncopated".
More simply, syncopation is a general term for a disturbance or interruption of the regular flow of rhythm; a placement of rhythmic stresses or accents where they wouldn't normally occur.
Syncopation is used in many musical styles, and is fundamental in black-influenced styles such as jazz, ragtime, Cuban music, funk, ska, reggae, rap, jump blues, progressive rock, extreme metal, breakbeat, drum'n'bass, dubstep amd minimalism. "All dance music makes use of syncopation and it's often a vital element that helps tie the whole track together". In the form of a back beat, syncopation is used in virtually all popular music.
Syncopation has been an important element of musical composition since at least the Middle Ages. For some musical styles, such as jazz and ragtime, syncopation is an essential part of their character.