What does cutoff frequency refer to in filter circuits?

Cutoff frequency is defined as the frequency at which a filter begins to significantly attenuate the input signal, marking the transition point between the passband (where signals are allowed to pass) and the stopband (where signals are attenuated). In practical terms, at cutoff frequency, the output signal is typically reduced to a certain level, often -3 dB of the maximum passband level, indicating that the signal power has dropped to half its value. This concept is critical in designing filters to meet specific requirements based on the desired frequency characteristics of a circuit.

Understanding cutoff frequency is essential for applications in audio, communication, and signal processing, where selective filtering is necessary. The other choices do not accurately capture this definition; for instance, amplification and functional limits described in other options do not convey the significance of the transition from passband to stopband associated with the cutoff frequency.