Vacuum:
Refers to the low pressure condition created in the suction line. Also refers to the cleaning process of sucking leaves, algae and other debris from the pool floor.
Valve:
A device fit into a pipe that can either obstruct/limit the flow of pool water, or permit flow in one direction only.
Multi-port valve: A type of valve that allows flow in a variety of directions (see multiple filter control valve).
Velocity:
The speed at which liquid flows from one place to another.