@PotomacBob
They listed "CNN, major news networks, New York Times and Washington Post.
I don't know whether "major news networks" includes cable news.
It does.
The actual term 'Mainstream Media' goes back at least to the 1960's, so far as politics in the US is concerned. It began to be used in a pejorative by Rush Limbaugh about 25 years ago, because at that time, the three broadcast networks, the large cable networks, and the NYT, LAT, and WAPO, all had a very bad tendency to report on something using exactly the same verbiage. Then FOX was sitting over on the side, reporting on the same event, without apparently following the memo that this is how they're supposed to report it.
One example of this from television and cable media goes back to 2000, when Bush picked Cheney. You had NINETEEN media talking heads all describe Cheney using the same WORD - gravitas.
At heart is the issue that what is the 'mainstream' now? As far as the definition the Hill put out, they're referring to: CNN, ABC (mostly, because ABC actually still does reporting at times), NBC, CBS, MSNBC, the NYT, WAPO, and LAT. The joke used to be that the networks were : Clinton News Network, Always Backing Clinton, Nothing But Clinton, Clinton Broadcast Service, and MSDNC. The point being that these were all supposed to be the voices of all of America - and they're all either left leaning or so far left even their right side is on the left.
The United States is very much in the situation that Ernest is describing in his most recent book. The United States is a divided country. Both sides of the media show the same picture - a reporter standing in front of a building on fire. One side says, "These are mostly peaceful protests.", while the other side says, "Out of control rioters are burning down businesses while the police are being ordered by the mayor to stand down."
In either case, neither the New York Post nor the Wall Street Journal appear to be included, nor Huffington Post nor Breightbart or any of the blogosphere. Not Twitter nor Facebook nor any of the online stuff.
WSJ is mostly trade, so they're not MSM. NYP has the circulation to count, but leans right and also tries to be more of a local New York Paper than the NYT, so their headlines frequently are about DeBlasio and Cuomo. HuffPo and Breitbart lean left and right, respectively. Twitter and Facebook aren't supposed to censor ANY content, otherwise they're publishers instead of platforms. (Which they do.)