The frequency range (or ranges) outside the passband of the filter where attenuation of signal is desirable. This region is normally specified at some attenuation value or values. The closer to the passband that attenuation is specified, the more complex the filter design becomes.
The area of frequency where it is desirable to reject or attenuate all signals, as much as is practical.
The part of the frequency spectrum that is subjected to specified attenuation of signal strength by a filter.