Just who or what is this Republican "establishment"?

Still unhealthily fixated on the US Republican primaries, I got to wondering just who or what this Republican establishment (or, increasingly, the capitalized Republican Establishment) actually is that I keep reading about, and that I talked about in a recent post on this very blog. I've a suspicion that many other people are probably in the same boat. This "establishment" seems to be a very faceless and protean entity, but an article in today's Globe and Mail has helped me, at least to some extent, understand what it meant mean.
We've all been reading about how the Republican establishment has been at first angry at non-establishment upstarts like Donald Trump and Ted Cruz, and then concerned, even worried, about Trump's continued domination in the polls. More recently, it has been more like resignation and denial, particularly as the only establishment candidate in the Republican primaries, Marco Rubio, has continued to fade. It's more a case of looking to how to get the party back after the election, not how to save it.
But what, then, IS this establishment?
Eric Andrew-Gee's article defines it as "a loose nexus of people with a stake in the fortunes of the Republican Party [who] still try to shape its agenda and strategies". That might still sound unavailingly vague and woolly, but it gives something of the flavour of the beast. That description would include most mainstream Republican politicians, current and past (heavy-hitters like Mitt Romney, Paul Ryan and Mitch McConnell have recently come out strongly against Trump); mainstream voters (there's that key word "mainstream" again); Super PACs that support the traditional Republican values of small government and free-market economies, but that also have a preference for dignified and statesmanlike behaviour in politics, and that care about the world's perceptions of the USA; and last, but far from least, the policy wonks who are at least nominally responsible for setting the rules, policies and campaign activities of the party as a whole, particularly the Republican National Committee.
Essentially, this amounts to what one might call Republican traditionalists, those who want to continue with the politics-as-usual that has seen the party enjoy substantial electoral success over the decades. They are also people who see extremists like Trump and Cruz - whether in the realm of immigration policy or religion - as a threat to the traditional order, and to the prospects of the party, and America itself, continuing to be successful and respected on the national and international stages.
Indeed, they are what one might even call "sensible" or "thoughtful" Republicans - however much one may disagree with their beliefs - as opposed to the impulsive, extreme and irrational wing represented by blood-and-thunder sensationalists like Mr. Trump and Mr. Cruz.