The cutoff frequency of an anechoic wedge or set of wedges is the lowest frequency above which the normal incidence sound absorption coefficient is at least .990
Plane wave propagation will occur in ducts up until ka~1.84
That frequency below which electromagnetic energy will not propagate in a waveguide.
The frequency at which the filter circuit changes from an action of rejecting the unwanted frequencies to an action of passing the desired frequencies. Conversely, the point at which the filter circuit changes from an action in which it passes the desired frequencies to an action in which it rejects the undesired frequencies.