A Passband is a specification of the range and strength of Wavelengths (and in effect, Frequencies) allowed through a Filter that cuts off both high and low wavelengths, i.e., a specification of the wavelengths allowed to pass through the filter. In effect, it is a Sensitivity Function (S). The term is also used to mean the filter itself.
The term is used in engineering disciplines such as communications engineering as well as astronomy. In the latter, a Photometric System, e.g., the UBV Photometric System, is a conventional set of such filter specifications.
The passbands/filters are generally indicated by letter, some being mnemonic for the color, e.g., B for Blue, R for Red, but the same letter may have differing specifications under different systems. Some common passband letters in astronomy:
Full Width At Half Maximum (FWHM)
Gas Fraction Estimation
Hawaii K-Band Galaxy Survey
JHK Photometric System (JHK)
Sensitivity Function (S)
Strömgren Photometric System
UBV Photometric System