For a filter or thin-film coating, the wavelength range over which transmission is allowed and possibly maximized. Transmission above or below the bandpass range is restricted by design through absorption and/or reflection.
Allowing a certain range or band of frequencies to pass through a system, while blocking or restricting others. In audio, this can be done electrically in a crossover, or mechanically as in a bandpass subwoofer box.
Specific range of frequencies allowed to pass through a device