Noun
Butterworth filter (plural Butterworth filters)
(signal processing) A kind of filter designed to have as flat a frequency response as possible in the passband.
The Butterworth filter has the poorest transition but has a more even response, avoiding ripples in either the passband or stopband. Source: Internet
On any Butterworth filter, if one extends the horizontal line to the right and the diagonal line to the upper-left (the asymptotes of the function), they intersect at exactly the cutoff frequency. Source: Internet