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also main-stream, main stream, "principal current of a river," 1660s, from main (adj.) + stream (n.); hence, "prevailing direction in opinion, popular taste, etc.," a figurative use first attested in Carlyle (1831). Mainstream media attested by 1980 in language of U.S. leftists critical of coverage of national affairs.
The prevailing current or direction of a movement or influence: “The candidate's speech represented the mainstream thinking on economic policy.”
Conventional; materialistic: It's a mainstream movie; don't waste your time (1980s+ Students)