Syncopation

Syncopation is a type of rhythm where some of the accents are "off the beat".
For example, a simple tune can be made to include syncopation by moving the position of one note back (in time) by half a beat, so making it longer, and the note before it shorter, as in the last note of this example:

The quaver-crotchet-quaver rhythm in the first bar is also a form of syncopation, but weaker than the one just described.
It can be caused by moving an accent forward or backward, by having a rest on an accent, or by having an accent where it is not expected.
Syncopation is often thought of as being a modern invention, being used exclusively in jazz and pop music. However, it exists in music of several centuries ago, including the music of Bach and Mozart.
It has become very popular in the last 100 years because of the strong influence from African music in dance music, rock and pop music.