Universal Doom Map Format
Universal Doom Map Format is a map format that is planned to be supported in Eternity. It is written in text mode, and like ExtraData, its purpose is to allow arbitrary definition of map objects (things, linedefs, sectors...), so that developers can add any new features without being limited to the fixed-size nature of the classic binary formats of Doom or Hexen mode.
The standard was defined by James "Quasar" Haley and has been successfully implemented in Vavoom, ZDoom and the ZDoom-derivative ports.
UDMF on Doom Wiki
UDMF on ZDoom wiki