Question

Mainstreaming can be defined as

Answer 1

It depends on which mainstreaming you are referring to. 
Mainstreaming can simply mean - to bring into the mainstream.
When it comes to education, here is what mainstreaming is: the practice of educating students with special needs in regular classes during specific time periods based on their skills.
There's also a thing called gender mainstreaming, which is a strategy for promoting gender equality.