Adjective

1. Very popular, as a fad; familiar to the masses; common, usual or conventional.

   They often carry stories you won't find in the mainstream media.

Noun

1. That which is common; the norm.

   His ideas were well outside the mainstream, but he presented them intelligently, and we were impressed if not convinced.

Verb

1. To educate (a disabled student) together with non-disabled students.

   Mainstreaming has become more common in recent years, as studies have shown that many mainstreamed students with mild learning disabilities learn better than their non-mainstreamed counterparts.

2. To popularize, to normalize, to render mainstream.