Cantofish: main stream (of a river); fig. the essential point; main viewpoint of a matter
MDBG: main stream (of a river); fig. the essential point; main viewpoint of a matter; mainstream (culture etc)
LDC:
CDict: (n) main stream of a fluid; (n) main aspect of a matter'
Click icons for complete source definitions (not available on mobile). icon in each entry gives source attribution. Want to improve this definition? Check to see if 主流 is already in CC-CEDICT. If not, you can add it. (Why?)