XFree86 Modeline
A modeline is a configuration line in xorg.conf or the XFree86 configuration file (XF86Config) that provides information to the X server about a connected computer monitor or television and how to drive it at a specified display resolution. The Modeline is based on the Generalized Timing Formula or the Coordinated Video Timings standards produced by VESA.
Modelines are now all but unused, but XFree86 and the Xorg Server still allow them to be set should the EDID information be inadequate.
Read more about XFree86 Modeline: Syntax, Obsolescence