UDMF (or unabbreviated to Universal Doom Map Format) is a specification for laying out maps in a textual way, it is essentially a new map format for Doom engine based games. The format is not backwards-compatible with older Doom engine games.
The idea for this format was first suggested by James Haley (Quasar) as a universal solution to add unlimited extra information to map elements, so that the wide range of new features in Doom source ports can be better supported by editors and implemented in the map information more easily. UDMF was further developed by Haley, Christoph Oelckers (Graf Zahl) and Pascal vd Heiden (CodeImp) and with suggestions of a few others. The name for the map format was coined by Russell Rice.
UDMF version 1.0 was finalized on May 29th, 2008. Support for it is currently implemented in GZDoom, Vavoom, ZDoom, Skulltag, ZDBSP, Doom Builder 2 and SLADE 3. It is also expected to be supported in Eternity and Doomsday.