In telecommunications, the term cutoff frequency f_c has the following meanings:

1. The frequency either above which or below which the output of a circuit, such as a line, amplifier, or filter, is reduced to the specified level of - 3 dB = 70.7 % of the reference voltage level of 0 dB = 100 %. See also time constant and center frequency. The calculation of the center frequency of such a frequency band is not the arithmetic mean but the geometric mean. See external link. [1]

A level drop of (-)3 dB means a voltage reduction to 70,7 % and also a power reduction to 50 %, because voltage V² is proportional to power P.
2. The frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer.

A bandpass has two cutoff frequencies and a center frequency.
See also: low-pass filter and high-pass filter.

External link

• Calculation of the center frequency with geometric mean and comparison to the arithmetic solution