Background

1. When referring to multitasking operating systems, a background is an active program currently running but not seen on the computer. For example, a user may open a CD or MP3 player and then minimize it and work on something else while still being able to listen to his or her music. Additional information and help with background programs or TSRs can be found on document CHTSR.
2. Background is also used to describe a Microsoft Windows computer wallpaper.

Also see: BG, Foreground, Operating System definitions, Thread