1.
main stream; main current of a river
The primary channel of a river, as distinguished from tributaries or branch streams.
2.
mainstream; orthodox line; dominant tradition
The dominant or orthodox position within a field, organization, or school of thought. Often used to distinguish the central tradition from offshoot or minority positions.
Composed of 本 (main; true) and 流 (current; flow). The geographical sense refers to the main channel of a river system, while the figurative sense describes the dominant tradition or orthodoxy in any field. In politics, 本流 often refers to the establishment or dominant faction within a party.