mainstream (Adjective) — Unimaginative and conformist. ex. "mainstream bourgeois lives"
mainstream (Adjective) — Very popular or familiar to the masses; conventional. ex. "They often carry stories you won't find in the mainstream media"
mainstream (Noun) — The prevailing current of thought. ex. "his thinking was in the American mainstream"
mainstream (Verb) — Put into the mainstream, esp. educate a special-needs student together with other students. ex. "Findings indicate that mainstreamed students with mild disabilities may participate in academic lessons as well or nearly as well as their peers"