Mainstream is, generally, the common current of thought of the majority. However, the mainstream is far from cohesive; rather the concept is often considered a cultural construct. It is a term most often applied in the arts (i.e., music, literature, and performance). This includes:

something that is available to the general public;
something that has ties to corporate or commercial entities.

As such, the mainstream includes all popular culture, typically disseminated by mass media. The opposite of the mainstream are subcultures, countercultures, cult followings, and (in fiction) genre. Additionally, mainstream is sometimes a codeword used for an actual ethnocentric or hegemonic subculture point of view, especially when delivered in a culture war speech. It is often used as a pejorative term. In the United States, mainline churches are sometimes referred to synonymously as "mainstream."

整個google從某權威source解釋的文章的結果都放在那了，斷什麼章了？除非是沒完全讀懂英文，那漏了(斷章) 就是自己的問題了。