noun

• (usually with the) That which is common; the norm.
  "ideas outside of the mainstream"

verb

• (transitive) To popularize, to normalize, to render mainstream.
  "Just as the gang peace movement desired to mainstream hardcore bangers into civic society, The Chronic wanted to drive hardcore rap into the popstream."
• (intransitive) To become mainstream.
  "In a nonchurch context, we can look more explicitly at formerly New Age practices to see if and how they have mainstreamed."
• (transitive, education, chiefly US) To educate (a disabled student) together with non-disabled students.
  "Despite these beliefs, the decision to send my son to a regular school was not made easily. I didn't know of any child as disabled as he who had been mainstreamed."