In philosophy of science:

Cosmic microwave background radiation, a natural constant radiation emanating throughout the universe

Background radiation, the natural radiation that is always present in a location

Background, or desktop picture, is the image or colour displayed on a website or computer desktop behind the visible content

Background independence, a condition in theoretical physics

Background (astronomy), small amounts of light coming from otherwise dark parts of the sky

Background (computer software), software that is running but not being displayed

Background lighting, a film technique

Background (album), an album by Lifetime

Operational definition, distinguishing an object from its background of observation, or identifying what is and what is not part of it