A Photometric System is a set of defined Passbands for Filters of known sensitivity to various Wavelengths of radiation. The term is generally used regarding Electromagnetic Radiation (EMR) in the Infrared (IR)-to-Ultraviolet (UV) range. With the specification of passbands, precise meanings of color indices are implied.
An early system was the Johnson-Morgan UBV Photometric System, which is generally assumed if no system is specified or implied. Equipment and scientific needs have prompted the development of new systems, and a major mission or survey (e.g., the Sloan Digital Sky Survey (SDSS)) often specifies its own.
The passbands/filters are generally indicated by letter, some being mnemonic for the color, e.g., B for blue, R Band for red, but the same letter may have differing specifications under different systems.
JHK Photometric System (JHK)
Sloan Digital Sky Survey (SDSS)
Strömgren Photometric System
UBV Photometric System