1. The frequency at which the attenuation of a passive network changes from a small value to a much higher value; this is the theoretical cut-off frequency. The effective cut-off frequency is that frequency where the insertion loss between two specified impedances has risen by a stated amount compared to the value at a reference frequency. An active network has the same cut-off frequency as a passive one with the same inductances and capacitances.
2. (break frequency; band edge; corner frequency; critical frequency) The point at which the angular frequency of a signal is equal to the reciprocal of the circuit time constant.
3. In a field-effect transistor used for microwave applications, that frequency at which the current gain of the FET becomes zero.