Mainstream was used first in English as the main part of a river back in 1585. In terms of etymology the word is closely related to the Old Icelandic meginstraumr or mighty stream. Milton uses it in Paradise Lost when he described the main river in the Garden of Eden being “divided into four main streams”.
The shift of the word’s meaning to mean a major trend came much later. Along these lines the word appeared in an 1831 article by Thomas Carlyle in the Quarterly Review, a journal with a mix of literary and political articles. It refers to a tendency that mingles with others, but still remains the mainstream. It was almost 100 years before the word moved away from its associations of flowing and rivers.
It was only 30 years ago that mainstream as a word was taken up as a description of the prevailing trend, as a way of describing popular culture, the thoughts of the majority. Now it is used as a way of describing great success, for internationally successful films or music and established science. And of course for anyone who is not in the mainstream, it is used in the pejorative, as a way of belittling mainstream culture.
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