noun
the principal or dominant direction or trend of a human activity, thought, or movement.
She is an innovator and her creations have always differed greatly from those of the mainstream.
adjective
sharing, agreeing with, or shaped by the prevailing trends and opinions of a group or society.
He was somewhat of a radical thinker in his youth, but his ideas have become more mainstream in recent years.
The big radio stations play fairly mainstream music.