Apparent magnitude, photometric band কী?

A photometric band is a range of wavelengths selected by a standard filter, named by a letter: U (ultraviolet), B (blue), V (visual), R (red), I (near infrared), and J, H, K in the infrared windows. The apparent magnitude $m$ of a source in a band is defined from its flux $F$ in that band by

with the zero point fixed by standard stars, close to the magnitude of the star Vega in each band. A difference of 5 magnitudes is a factor of exactly 100 in flux; one magnitude is a factor $100^{1/5} = 2.512$. Brighter sources have smaller, and possibly negative, magnitudes.