From bowiki

Adapted From Wikipedia

In music, syncopation includes a variety of rhythms which are in some way unexpected in that they deviate from the strict succession of regularly spaced strong and weak beats in a meter (pulse). These include a stress on a normally unstressed beat or a rest where one would normally be stressed. "If a part of the measure 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.