All about DEF Files

The DEF file type is primarily associated with Notepad.

DEF	File extension: DEF
	File type:	Plain text definition file

What is a DEF file?

DEF files mostly belong to Notepad. Your DEF file is a plain text definition file used by various software systems to specify configuration, exported symbols, or game parameters. Its content is not universal – it changes based on the tool that uses it.
Here are some common uses:

M.U.G.E.N Definitions: These files define game elements such as fighters, stages, and character parameters for the M.U.G.E.N engine. For example, a file might list a character's move set or stage details. Additional insights can be found at this resource.
Module Definition Files: In software development, especially in Visual C++ projects, a DEF file lists exported symbols for linking dynamic libraries. It may be UTF-8 encoded and follows a strict syntax so that linkers like LINK.EXE know what to export. See Microsoft documentation for details.
Other Game & Application Definitions: Some DEF files serve as configuration or data files in game development (for titles such as Dungeon Siege or Men of War) and even in specialized applications like Fighter Factory or accounting systems (e.g. Abila MIP import files, more info at Abila MIP).
Design Exchange Format: In some cases, the file is used in design environments – for example, Cadence Innovus uses a Design Exchange Format DEF (Cadence) to manage layout or design data.

FilExt.com note: Most DEF files are plain text and can be opened with any text editor (for instance, Notepad). However, editing them correctly usually requires familiarity with the particular system’s syntax and conventions, as changes can affect how a module is linked or how a game element is defined.

How to open DEF files

You need a suitable software like Notepad to open a DEF file. Without proper software you will receive a Windows message "How do you want to open this file?" or "Windows cannot open this file" or a similar Mac/iPhone/Android alert. If you cannot open your DEF file correctly, try to right-click or long-press the file. Then click "Open with" and choose an application. You can also display a DEF file directly in the browser:. Just drag the file onto this browser window and drop it.

Online DEF Text Viewer

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Programs that open and convert DEF files:

Notepad
See the previous paragraphs to learn more about the main application. DEF files are often referred to as Notepad games because this type of file is primarily created or used by this software.
3D Fassade Plus
ADS (Source Code) by Autodesk, Inc.
AniSprite by CompuPhase
ArcView (Defaults File) by ESRI
ODB Format (North Arrows, Layout Templates, etc) Related links: GIS Info
Bert's Dinosaurs File
C++ Definition
Data Entry/Review Module by DATAIR Employee Benefit Systems, Inc.
Defines Data Entry forms.
DbForm (Document Definiton) by Version One Ltd
DbForm is an enterprise-wide white-paper printing solution, which automatically prints high quality business documents directly from core business applications onto plain paper on laser printers. This file is a script describing the creation of a PCL document.
Defaults
Definitions
Geoworks
Modula-2 Definition Module File
Module Definition/Configuration File
OPTLINK (Digital Mars Module Definition File) by Digital Mars
OPTLINK, the Digital Mars linker, links object files and libraries to produce an executable program or DLL for DOS, Windows 3.x, or Win32. The .DEF file describes the essential characteristics of an application or library for Win16 or Win32. It also assigns segment sequences, function orders for encapsulated functions, and so on, even in conventional DOS .EXE files. Related links: Module Definition Files Described
RCSWin (New (Default) Job File) by RCS Corporation
RCSWin is a program for mechanical design, analysis, and cost estimating of shell-and-tube heat exchangers.
SGML Tag Definition
The Mime types used with this association are: text/plain.
SmartWare II Data File
TLink - Wlink - Others
Visual C++
ZDP ZEBU's Directory Printer (Definitions of User-defined Output Formats) by ZEBU Informatics
With ZDP you can export directory entries to other programs or to the clipboard. Related links: ZDP Help, FAQ

DEF file format:

The file format determines how the data is stored in the file. The initial characters of a file are called signature, sometimes also referred to as "magic bytes". The signature can be used to infer the file format. Nonetheless, different programs can utilize the same file extension to represent distinct file formats. Double-clicking on the file often results in an error when opening. So a detailed analysis of a file is vital in order to determine the format and the associated program. In the following you will find our evaluation of the DEF files:

The DEF file extension does not have a uniform file format. But the following two file formats are common:

6% of all DEF files use the same file format, which can be identified by the letters ";----------------------------" at the beginning. The contents of these files are easily readable with a text editor. DEF files are between 770 bytes and 6 KB in size. The keywords ;-----------, [Characters], ;How to add characters, ;Use the format, ; charname, stagefilename and ;eg. For are typical for these files. A common filename is select.def.
10% of all DEF files are based on the well-known config file format. A configuration file is used by computer software to store settings. It consists of a text-based content with a structure and syntax comprising key–value pairs for properties, and sections that organize the properties. These files are text files and can therefore be read using a text editor. A typical file size is 940 bytes. The files contain the following words: ;******************* http://www.virtu and ;******************* Created in Fighter Factory Ultimate *******************;.

All other DEF files (84%) have different formats, the following 6 formats can be found: UTF-8 and REM. Often described as config, intro, application, level, game or info. You can get all the details of your DEF file by dragging it onto this page or by clicking the button "Choose your .def file to analyze" above.

Technical Data for DEF File Extension

File classification:

Game

Related files:

exp, lib, pdef, sff, actiondef, cns, tgdef, 82m, tp6, rar, scs, zip, lef, txt, 3da, cmd, makefile, air, sprite, cnv, pdf, act, fld, d2d, ini, cfg, dll, obj, dat

Mime type:

text/plain, application/octet-stream, text/x-modula2

The following listing is compiled from the database produced by the 'Associate This!' program, selected data from the main FILExt database and file extension information submitted by users.

ProgramID: deffile, FileType: Export Definition File, AppName: Microsoft Visual Studio .NET 2002
EXEFile: devenv.exe

ProgramID: deffile
EXEFile: %ProgramFiles%\Microsoft Visual Studio .NET\Common7\IDE\devenv.exe /dde

ProgramID: RVS-DEF
EXEFile: %ProgramFiles%\RVS\WCOM\SYSTEM\RVSEDIT.EXE %1

ProgramID: VisualStudio.def.7.1
EXEFile: %ProgramFiles%\Microsoft Visual Studio .NET 2003\Common7\IDE\devenv.exe /dde

ProgramID: VisualStudio.def.7.1
EXEFile: %ProgramFiles%\Visual Studio .NET 2003\Common7\IDE\devenv.exe /dde

ProgramID: VisualStudio.def.8.0
EXEFile: %ProgramFiles%\Microsoft Visual Studio 8\Common7\IDE\devenv.exe /dde %1

ProgramID: VCExpress.def.8.0
EXEFile: %ProgramFiles%\Microsoft Visual Studio 8\Common7\IDE\VCExpress.exe /dde %1

ProgramID: VCExpress.def.8.0
EXEFile: %ProgramFiles%\Microsoft Visual Studio 8\Common7\IDE\VCExpress.exe /dde

ProgramID: WinEdt.def
EXEFile: %ProgramFiles%\WinEdt Team\WinEdt\WinEdt.exe %1

ProgramID: VisualStudio.def.9.0
EXEFile: %ProgramFiles%\Microsoft Visual Studio 9.0\Common7\IDE\devenv.exe

ProgramID: VCExpress.def.9.0
EXEFile: %ProgramFiles%\Microsoft Visual Studio 9.0\Common7\IDE\VCExpress.exe

Show all 11 listings To Top

a def Plain text definition file is a special file format and should only be edited and saved with the appropriate software.

How to solve problems with DEF files

Associate the DEF file extension with the correct application.
Update your software that should actually open plain text definition files. Because only the current version supports the latest DEF file format. Search, therefore, e.g. on the manufacturer website after an available Notepad update.
To make sure that your DEF file is not corrupted or virus-infected, get the file again and scan it with Google's virustotal.com.
Click here to open your .DEF file online - secure, fast, and no downloads needed.