Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not innately attract matter. Dust is “sucked” into a vacuum cleaner when it is pushed in by the higher pressure air on the outside of the cleaner. The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter.
This is similar to what happens when humans breathe or drink through a straw. Both breathing and using a straw involve contracting your diaphragm and muscles around your rib cage. The increased area in your chest cavity decreases the pressure inside creating an imbalance with the ambient air pressure, or atmospheric pressure. This imbalance results in air being pushed into your lungs or liquid being pushed up into a straw and into your mouth.
Pumps typically have an inlet where the fluid enters the pump and an outlet where the fluid comes out. The inlet location is said to be at the suction side of the pump. The outlet location is said to be at the discharge side of the pump. Operation of the pump creates suction (a lower pressure) at the suction side so that fluid can enter the pump through the inlet. Pump operation also causes higher pressure at the discharge side by forcing the fluid out at the outlet. There may be pressure sensing devices at the pump’s suction and/or discharge sides which control the operation of the pump. For example, if the suction pressure of a centrifugal pump is too low, a device may trigger the pump to shut off to keep it from running dry; i. e. with no fluid entering.