, the term cutoff frequency
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 a specified level.
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.
Source: from Federal Standard 1037C and from MIL-STD-188