Who knows the definition, but I can tell you who is included. ABC, NBC, CBS, CNN.... Watching the anchors on these channels respond to Trumps performance on election night was jaw dropping. I was amazed that they weren't even going to try to pretend that they were impartially covering this election. I've always known the Fox and msnbc were a joke, but I have zero confidence in a single US based news outlet after this election.
BTW Huffington Post stands out as an exceptionally huge steaming pile of partisan garbage as well.
Both of those are left wing outlets.
I'd think "mainstream" would be the ones which still posit themselves as un-biased (CBS, NBC, ABC, NY Times, Wash Post, etc.).
I say the more the merrier. Let people decide.
There were a handful of news outlets thoroughly reporting issues between DNC and Bernie Sanders such as Huffington Post and al-jazeera.
I suppose these are not 'mainstream'? Perhaps mainstream media is defined by having a dedicated TV channel, or at least primetime TV show? Isnt it almost customary for such outlets to prioritize ratings over comprehensive coverage?