Syncopation
Syncopation is the regular accenting or emphasising of notes that would usually not be. The time signature of a piece of music gives an indication of a regular rhythm pattern of strong and weak beats, whereas a syncopated rhythm goes against this expectation by putting accents off-beat or on weak beats. In beatmapping, syncopation in a song can usually be seen when hit objects are regularly put off the beat instead of on the beat, such as on red or blue timeline ticks.
As a simple example, fanzhen0019's Insane difficulty of Chasers - Lost contains slider patterns both with on-beat and off-beat sliderheads.
Some of the slider patterns, such as the very first one, follow the synth melody which is syncopated against the drum kicks, landing on red timeline ticks:
This is contrary to slider patterns in the same beatmap that follow the percussion, landing on white timeline ticks: