Another definition adds some other dimensions – who is "mainstreamed" with what expectations and which supports.

Mainstreaming was a term popularized after the passage of P. L. 94-142 which has been generally used to describe the process of placing a student with mild to moderate disabilities into one or more general education academic classes. Students who are mainstreamed are usually expected to meet the same standards as students without disabilities with minor modifications in curriculum or methodology. Prerequisite skills are generally felt to be necessary since the same standards for success are being applied for all students. This delivery model identifies the child as a 'special' rather than a 'general' education student. This practice has not typically been associated with students who are identified as having severe disabilities.

Looking out a window in a classroom
Photo courtesy William Bronston, M.D.