Cutoff means to shut off or intentionally stop. Cutoff frequency is defined as a boundary between the pass band and transition band of a signal in a filter circuit.

Filter circuit allows only signals in a specific frequency band to pass through and attenuates signals in other frequency bands, and it is used to remove noise from the received signal. In other words, frequencies in the passband are accepted as signals, and signals with frequencies higher than the cutoff frequency are attenuated as noise.

The transition between the passband transition band is a continuous function. Therefore, the cutoff frequency is treated as one of the most important values for the characteristics of the filter circuit.