X Bitmap. This file format is a C declaration of an unsigned char array and two #defines:

#define filename_width 16
#define filename_height 16
unsigned char filename_bits[] = { 0x0, 0xCF, ... };

Compare xpm, which is ASCII-based.
Y'know, if you log in, you can write something here, or contact authors directly on the site. Create a New User if you don't already have an account.