By extension, the non-data bits in any self-describing format. "A GIF file contains, among other things, green bytes describing the packing method for the image".
At a meeting of the SHARE Systems Division, November 22, 1964, in Washington, DC, George Mealy of IBM described the new block tape format for FORTRAN in which unformatted binary records had a Control Word. George used green chalk to describe it. No one liked the contents of the Green Word (not information, wrong location, etc.) so Conrad Weisert and Channing Jackson made badges saying "Stamp out Green Words". This was the first computer badge.