throughput — Throughput refers to the pumping speed multiplied by the gas pressure at the inlet, and is measured in units such as torr-litres/second. At a constant temperature, throughput is proportional to the number of molecules being pumped per unit time, and therefore to the mass flow rate of the pump. (Think PV=nRT) When discussing a leak, backstreaming or outgassing, throughput refers to the volume leak rate multiplied by the pressure at the vacuum side of the leak, so the leak throughput can be compared to the pump throughput.

References: Wikipedia, the free encyclopedia – http://en.wikipedia.org/w/index.php?title=Vacuum_pump