G band (G for "green") is sometimes a designator sometimes given to visible light bands (roughly) corresponding to the color green, i.e., whose wavelengths are in the 510 nm region. Sloan Digital Sky Survey data contains information from a G band that appears to be SDSS nomenclature (termed g') that covers 400 nm to 540 nm (including the color green but considerably more as well).
The term G band (more specifically, Fraunhofer's G band) is also commonly used to designate a (non-green) band covering the range of 430.40 nm to 430.61 nm, in which Sun light has a number of absorption lines which are among the Fraunhofer lines. Lines in Fraunhofer's G band include iron and methylidyne lines, which are prominent in G-type and K-type stars.
The term G band is also used with other specifications in radio communication.