Mainstream media (MSM) are those media disseminated via the largest distribution channels, which therefore represent what the majority of media consumers are likely to encounter. The term also denotes those media generally reflective of the prevailing currents of thought, influence, or activity.
Large news conglomerates, including newspapers and broadcast media, which underwent successive mergers in the U.S. and elsewhere at an increasing rate beginning in the 1990s, are often referenced by the term. This concentration of media ownership has raised concerns of a homogenization of viewpoints presented to news consumers. Consequently, the term mainstream mediahas been widely used in conversation and the blogosphere, often in oppositional, pejorative, or dismissive senses, in discussion of the mass media and media bias. http://en.wikipedia.org/wiki/Mainstream_media