The principal current in a flow, such as a river or flow of air
"The boat drifted slowly downstream until it entered the fast-moving mainstream of the river."
In plain English: Mainstream refers to what most people generally do, think, or follow at any given time.
"The new fitness trend quickly became part of mainstream culture, with people all over the city joining in."
To popularize, to normalize, to render mainstream.
"The documentary succeeded in making climate change a mainstream topic of conversation across the nation."
In plain English: To mainstream something means to make it fit into what most people normally do or accept.
"The new trend quickly became mainstream after everyone started following it on social media."
Used or accepted broadly rather than by small portions of a population or market.
"The once-niche art style has become mainstream as galleries and museums now widely exhibit it."
In plain English: Mainstream describes something that is popular and accepted by most people rather than being unusual or extreme.
"The new technology quickly became mainstream and is now found in most smartphones."
Usage: Use "mainstream" as an adjective before nouns like music or culture to describe trends accepted by the general public rather than niche groups. Avoid confusing it with specific subcultures when referring to widely established norms and values in society today.
From main + stream. Cognate with Icelandic meginstraumur ("a main current, mainstream").