A gauge (flow instrument) reads and displays the value of a particular property of a system at a point, or displays the difference in the value of a property between two points, at that point in time. A gauge does not record or integrate the values over time (although it may be connected to recording devices that do perform such a function). In this instance a gauge can measure mechanical flow or electrical flow.
A frequency meter is a type of gauge that reads and displays the electrical frequency of an alternating current circuit.