app: /ap/ n. Short for `application program', as opposed to a systems program.

:app: /ap/ n. Short for `application program', as opposed to a
systems program. Apps are what systems vendors are forever chasing
developers to create for their environments so they can sell more
boxes. Hackers tend not to think of the things they themselves run
as apps; thus, in hacker parlance the term excludes compilers,
program editors, games, and messaging systems, though a user would
consider all those to be apps. (Broadly, an app is often a
self-contained environment for performing some well-defined task
such as `word processing'; hackers tend to prefer more
general-purpose tools.) Oppose {tool}, {operating
system}.
-- The AI Hackers Dictionary