A photometric system is a set of defined bands for filters of known sensitivity to various wavelengths of radiation. The term is generally used regarding electromagnetic radiation in the infrared-to-ultraviolet range. With the specification of passbands, color indices have precise meanings.
An early system in common use 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) often specifies its own.
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.