Bak file
In computing, ".bak" is a filename extension commonly used to signify a backup copy of a file.

When a program is about to overwrite an existing file (for example, when the user saves the document they are working on), the program may first make a copy of the existing file, with .bak appended to the filename. This common .bak naming scheme makes it possible to retrieve the original contents of the file in case of a failed write that corrupts the file, which could be caused by an operating system crash, power outage, or disk space exhaustion.

Without the backup file, an unsuccessful write event may truncate a file, meaning it cuts off the file at a position, or leaves a blank file. In practice, this could cause a written document to become incomplete or get lost, a multimedia project file (e.g. from a video editor) to become unparseable, and user preferences being reset to default.

In a similar manner, a user may also manually make a copy of the file before the change and append .bak to the filename, or alternatively save revisions into separate files, to facilitate reverting to an earlier revision in case of an error.

Other naming schemes are also in widespread use: file~, file.orig, file.old, and appended time stamps.

Database Applications like FoxPro and SQL Server use .bak files to back up their databases and other applications, like XML shell, create .bak files in their autosave process. They do not get automatically deleted, so they need to be manually deleted after the process using it is stopped.

Example applications using .bak files 

This is a partial list of applications that generate .bak files (in some cases as an optional configuration setting):
 Apophysis7X
 Alteryx
 Aspell
 Audacity
 AutoCAD
 fluidsim
 BlueGriffon
 Captivate
 Cubase
 DraftSight
 DrRacket
 Exiled Kingdoms rpg 
 Family Tree Maker
 Finale Notepad
 FoxPro
 FormZ
 Google Chrome
 Grand Theft Auto V
 HxD
 HyperCam
 JavaFX Scene Builder
 Lazarus
 LMMS
 MATLAB
 Minitab
 Mozilla Firefox
 Mupen64
 Notepad++
 NUPS
 Opera
 Outlook Express
 Password Safe (.ibak)
 Photoshop
 PScad
 Free Pascal
 Rhinoceros 3D
 Sony Vegas
 SQL Server
 SCIA Engineer
 Steinberg Cubase
 Starbound
 TeamViewer Manager
 Terraria
 Texmaker
 TextCrawler
 Turing
 Turbo C++
 UltraEdit
 VEE
 WhatsApp
 Windows (BOOTSECT.BAK)
 WinMerge
 Word
 Zmodeler
 OpenMPT

External links 
 BAK file

References

Filename extensions