In simple terms, relative velocity is the difference between the speeds of two moving bodies. It can also be the differential velocity between solid particles, dust and/or liquid droplets in a gas flow (Venturi nozzle, Venturi scrubber and highway effect).

The higher this relative velocity/velocity difference, the better the potential separation in a Venturi scrubber. In the case of very fine and light particles, other physical effects can come into play that can override the impact of relative velocity in separating dust.