A mechanism of samplers and sample playback synths in which, when a given note is played, the actual sample that sounds depends not only on the note played but also the velocity. Depending on the capabilities of the sampler, as an example, it might divide the range of possible velocities into four bands, and choose a sample to play based on which band the velocity value falls into.
Velocity switching is most often used in imitative synthesis to more accurately reproduce the sounds of accoustic instruments whose tonality varies depending on how hard or loudly they are played. By sampling the instrument at several volume levels, and then mapping each sample to an appropriate band of velocity values, the sample can realistically reproduce the sound of the instrument at different playing volumes. Compare to multisampling.