百度了下，貌似后补性媒体就是Alternative media。
Alternative media are media (newspapers, radio, television, magazines, movies, Internet, etc.) which provide alternative information to the mainstream media in a given context, whether the mainstream media are commercial, publicly supported, or government-owned. Alternative media differ from mainstream media along one or more of the following dimensions: their content, aesthetic, modes of production, modes of distribution, and audience relations.
