The term 'background' refers to features of the universe, external to the part of the universe being studied, which may nevertheless be necessary to include in order to define the properties of the part of the universe under consideration. For example, in Newtonian physics, space and time are part of the background because they are taken to be absolute. Hence, Newtonian physics is said to be background dependent. In contrast, a theory that does not make use of a division of the universe into a part that is modelled and the rest, which is taken to be part of the background, is called background independent. General relativity is background independent because, in it, the geometry of space and time is not fixed, but evolves in time just as any other field, such as the electromagnetic field, does.