DEF (C++ Module-Definition File)
The DEF Developer file format
Files that have the DEF extension can be associated with several programs. One of the most common is Microsoft Visual Studio, and more precisely, Microsoft Visual C++. Other C++ development programs can create DEF files as well though.
Technical details of DEF files
DEF files are saved in a plain text format which allows them to be opened in any text editor across different operating systems. The DEF file lists different attributes of a program in order for an application linker to be able to correctly link them. These files are mostly used when creating DLL files in software developing.