The Collaborative International Dictionary of English v.0.48:

mainstream \main"stream`\ n.
   The prevailing opinion or practise; as, the doctor avoided
   using therapies outside the mainstream of modern medical
   practice.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

mainstream \main"stream`\ v. t. (Education)
   TO place (a student) in regular school classes; -- used
   especially of mentally or physically handicapped children.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

mainstream
    n 1: the prevailing current of thought; "his thinking was in the
         American mainstream"

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "mainstream":
   Zeitgeist, bearing, course, current, direction, drift,
   glacial movement, line, main current, motion, movement, run, set,
   stream, swing, tenor, the general tendency, the main course,
   time spirit, tone, trend