In a Western context, the term “mainstream media” is most often understood to refer to established, traditional forms of communication — such as mass-circulation newspapers and magazines, television and radio — in contrast to alternative forms of communication like the Internet and other new media, or publications with a less mainstream reach or agenda. See, for example, this article from Poynter.org on the “mainstream press” versus Wikipedia.
In China, the term “mainstream media” has a very different sense, and usually refers to established, party-run media — the likes of People’s Daily, China Central Television and provincial party media — that have typically served a stricly “mouthpiece” role in disseminating the CCP line. In this sense, the more commercially oriented newspaper and magazines that have emerged in China since the 1990s are not regarded as “mainstream” and so are “non-mainstream,” or fei zhuliu (非主流).
Of course, as all media are ultimately controlled by the party in China, the voices in non-mainstream media (with some notable exceptions) are most often more palatable versions of the the party mainstream, and all media are subject to the dictates of propaganda discipline and “guidance.”