/ap/
n. Short for 'application program', as opposed to a systems program.
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.
Oppose tool, operating system.