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What is a .def file?
A .def file is an ESRI Arc/View Project (version 3.x). This one is a specialist format without an obvious everyday home. Confusingly, this extension has been used by 2 different formats over the years — they're all listed below.
This extension has been used by more than one format
2 different formats have claimed .def over the years, so two files with the same extension aren't necessarily the same kind of thing.
How to open a .def file
- The program that created it is by far the most reliable way to open it.
- On Windows, use a hex viewer or the free TrID tool to identify what the file really is.
- On Mac or Linux, running `file yourfile` in the terminal identifies most formats instantly.
The registry records this extension against ESRI Arc/View Project, Folio Definition File, so software from that family is the most likely match.
How to convert a .def file
- Identify the file first, then look for an export or Save As option in whatever program can read it.
The technical record
For archivists, developers and the merely curious: these are the PRONOM registry entries for .def.
| Format name | Version | MIME type | PUID |
|---|---|---|---|
| ESRI Arc/View Project | 3.x | — | fmt/332 |
| Folio Definition File | — | — | fmt/1163 |