DEF files are native to M.U.G.E.N game engine. DEF file is a text file containing key information about a game character. DEF document also stores references to other files, such as AIR, CNS, or SFF. The game eingine is distributed under freeware license and is no longer ectively deped by Elecbyte, the engine’s creator.

DEF files created by users can be found on Mugen-related game forums. The engine is still used and developed by the fan comunity.