the principal or dominant course, tendency, or trend: the mainstream of American culture.
2.
a river having tributaries.
3.
regular school classes or regular schools: keeping autistic students in the mainstream.
adjective
4.
belonging to or characteristic of a principal, dominant, or widely accepted group, movement, style, etc.: mainstream Republicans;a mainstream artist;mainstream media.
5.
of, pertaining to, or characteristic of jazz falling historically between Dixieland and modern jazz; specifically, swing music. Compare traditional( def 4 ).
"principal current of a river," 1667, from main (adj.) + stream, hence, "prevailing direction in opinion, popular taste, etc.," a fig. use first attested in Carlyle (1831).