Calibrated orifice
A calibrated orifice is a restriction that is deliberately placed into a system of pipes to set the flow rate through the system. The orifice may be designed to produce proportional flow (as in the jet in a carburetor), or choked flow (as in a filtering bypass in a closed industrial cooling system, which might be designed to pass a particular flow rate through a filter assembly to maintain cleanliness of a closed-loop fluid system).
Many pressure gauges also use an orifice (also called a restrictor) to limit the flow into a gauge. Since the pressure is even throughout the system, allowing only a small portion of the flow into the actual gauge allows it to be in parallel with the pressure circuit and still measure accurately. It also prevents or minimizes damage to the gauge during pressure surges at start-up, or due to any spikes in the system pressure.