Definitions for "Butterworth filter"
A filter with a pass-band with no ripple but usually sacrifices some steepness in attenuation.
A filter network that exhibits the flattest possible response in its passband. The response is monotonic, rolling off smoothly at the rate of 6dB per octave, per pole.
A type of active filter characterized by a constant gain (flat response) across the midband of the circuit and a 20 dB per decade roll-off rate for each pole contained in the circuit.