APP is an abbreviation for application

Application software

Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...

. The filename extension .app means application in Symbian OS, SkyOS

SkyOS

SkyOS was a prototype commercial, proprietary, graphical desktop operating system written for the x86 computer architecture. As of January 30, 2009 development has halted and no plans to resume its development have been announced.- History :...

, GNUstep

GNUstep

GNUstep is a free software implementation of Cocoa Objective-C libraries , widget toolkit, and application development tools not only for Unix-like operating systems, but also for Microsoft Windows. It is part of the GNU Project.GNUstep features a cross-platform, object-oriented development...

 and Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

.
On the latter two systems, a .app file is actually a folder containing binaries and supporting files.

App may also refer to a lightweight application, tailored to a device's form factor and input-output interfaces (key pad, touch pad, remote control, display, etc). Devices may include such things as a mobile phone, a TV set, a cable or satellite receiver, a tablet PC, a printer - any device whose features can be extended through software.

See also

• Application bundle (in Mac OS X
  Mac OS X
  Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
  
   and GNUstep
  GNUstep
  GNUstep is a free software implementation of Cocoa Objective-C libraries , widget toolkit, and application development tools not only for Unix-like operating systems, but also for Microsoft Windows. It is part of the GNU Project.GNUstep features a cross-platform, object-oriented development...
  
  )
• Mobile application development
  Mobile application development
  Mobile application development is the process by which application software is developed for small low-power handheld devices such as personal digital assistants, enterprise digital assistants or mobile phones...
  
  
• SIS (file format)
  SIS (file format)
  SIS is an acronym that stands for Software Installation Script. It is an archive for Symbian OS, and not an application file, as sometimes believed; the real Symbian application is the .APP or .EXE file within it. By convention .sisx denotes a signed file....
  
  , Symbian installation source for APP and other files
• Symbian OS
• Symbian Ltd.
  Symbian Ltd.
  Symbian Ltd. was a software development and licensing company, known for the Symbian OS, a smartphone operating system, and other related technologies...
  
  
• Super OS#Software repository (RUNZ)