A circuit which is used to either block or reduce a range of frequencies. There are lowpass filters, which pass frequencies below a certain point, called the "cutoff frequency", highpass filters, which pass frequencies above the cutoff frequency, bandpass filters which pass frequencies above a lower cutoff frequency and below an upper cutoff frequency, bandstop filters, which pass frequencies below a lower cutoff frequency and above an upper cutoff frequency, and allpass filters, which pass all frequencies at the same amplitude, but which have certain phase or delay characteristics.