6 file types use the .def file extension.

What is a DEF file?

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.

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Common DEF Filenames

How to open a DEF file

Category: Game Files

Module-Definition File

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.

Programs that open DEF files

Updated: 2006

SmartWare II Data File

A DEF file may also be a data file used by SmartWare II, a discontinued database and office productivity suite. It contains structured data that SmartWare II displays in tables, lists, or forms, allowing users to manage and view records and other entries.

SmartWare is an office suite with database, word processor, and spreadsheet programs. It was initially developed in the 1980s for MS-DOS and Unix, and later Microsoft Windows, and used in office environments. It is now discontinued.

Programs that open DEF files

Updated: October 17, 2024

IOBit Malware Fighter Definitions File

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.

Programs that open or reference DEF files

Updated: October 1, 2012

Kotlin C-Interop Definition File

A DEF file may also be a definition file that facilitates interaction between the Kotlin programming language and C libraries. It stores information that defines how Kotlin code should communicate with native C functions , structures, and constants . By providing this mapping, DEF files allow Kotlin developers to use existing C libraries in their Kotlin/Native projects without writing complex code that connects Kotlin with C.

Developers working on cross-platform applications, low-level system programming, or performance-sensitive tasks are the most likely to encounter DEF files. Kotlin uses these files in combination with the cinterop tool, which reads the DEF file and generates Kotlin bindings for the C library.

A DEF file typically specifies the library's name, header files, compiler options, and package structure for the generated Kotlin code. These details enable integration between Kotlin and C, making it easier for developers to reuse native libraries.

You can open DEF files with any text editor, such as Microsoft Notepad , Apple TextEdit , and Microsoft Visual Studio Code . Since DEF files control how Kotlin interacts with a C library, you should be careful when modifying them to avoid breaking the integration.

Programs that open DEF files

Updated: September 25, 2025

WWE 2K18 and 2K19 PAC Pointer File

A DEF file may also be a game data file used by WWE 2K18 and 2K19. It contains pointers to the .PAC files the games load at startup. The games' DEF file is named Chunk0.def .

WWE 2K is a series of popular video games based on the WWE pro wrestling franchise. WWE 2K14 to 2K19 were developed by Yuke's.

2K18 and 2K19 use an index file named Chunk0.def to note what PAC asset archives they should load at startup. This file is a plain text file, which most players will never need to open or edit. However, WWE 2K18 and 2K19 modders who have added custom PAC files to the games must add a reference to those PAC files to Chunk0.def . Otherwise, the games will not load the custom PAC files.

NOTE: Other Yuke's-developed WWE 2K games do not use a DEF file.

Chunk0.def - The DEF file included with WWE 2K18 and 2K19.

You can open and edit Chunk0.def with Microsoft Notepad (Windows) or any other text editor. Some WWE 2K modding tools, such as Custom Character Tools 2019 and Custom Character Tools 2018, allow modders to automatically update Chunk0.def after they've added a custom PAC to the games.

Programs that open or reference DEF files

Updated: April 3, 2023