Production music or library music is the name given to music which is produced exclusively for use in film, television, radio, internet and other media.

Unlike popular and classical music publishers, production music libraries own all of the copyrights of their music, meaning that it can be licensed without seeking the composer's permission, as is necessary in licensing music from normal publishers. This is because virtually all music created for music libraries is done on a work for hire basis. Production music is therefore a very convenient medium for media producers: they can be assured that they will be able to license any piece of music in the library at a reasonable rate.

Production music libraries will typically offer a broad range of musical styles and genres, enabling producers and editors to find much of what they need in the same library. Music libraries vary in size from a few hundred tracks up to many thousands.

Source: Wikipedia