.DEF File Extension

A DEF file is a game data file used by M.U.G.E.N (or just Mugen), a free sprite-based game engine used for making 2D fighting games. It contains the definition for a character, which includes the character name and version information. DEF files also store the character specification files (.CNS, .SFF, .CMD, .SND, and .AIR) used by the engine.

You will most likely only encounter a DEF file if you create games with the M.U.G.E.N engine. DEF files that store information for characters are located in the chars/ directory, while DEF files that store system-related information are located in the data/ directory.

While DEF files are referenced by M.U.G.E.N and are not meant to be opened, you can open DEF files with a text editor since they are saved in plain text. Some text editors you could use in Windows are Microsoft Notepad and Microsoft WordPad. In macOS, you can use Apple TextEdit.

Lists the exports and attributes of a program to be linked by an application linker; used in software development when creating a DLL.

DEF files are used by Microsoft Visual C++, a component of Microsoft Visual Studio, as well as other C++ development software.

Spreadsheet and database format used by SmartWare II database software.

File used by IObit Malware Fighter, a program used for removing malware and spyware from the PC; saves virus definitions used for identifying and removing malware from the computer.

DEF files use the filenames core000.def, core001.def, core002.def, etc. They are stored in the \db\ directory of the IObit Malware Fighter installation.