mainstream (n.)youdaoicibaDictYouDict[mainstream 词源字典]
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.[mainstream etymology, mainstream origin, 英语词源]