Ok, I must add my 2 cents to this conversation...hell, my 25 cents!.

A REGIONAL AirLINE is a somewhat small to mid size operator (amount of planes, capacity in those planes, and some might say total yearly revenue), that concentrates mainly in a REGION. Their original intent as part of their operations is/was to serve a specific REGION. Comair's is/was the northeastern region, ASA's the southeast, Skywest's the central/west, etc., etc., etc.
Now some of them thru the years have grown to EXPAND their REGION due to better/bigger/modern aircrafts (Brasilias vs 700/900, Saabs vs ERJ, etc.), but the intent still remains the same. They still serve a REGION, although that my be a larger one at this point (i.e. Republic, Compass, actually pretty much all of them). But one thing still remains constant, the serve a REGION. You may have an occassional crossing to Canada, Mexico, Bahamas, but they don't cross the pond on any direction with 150+ passengers. They are limited and/or concentrated in the lower 48.

Now, a REGIONAL AirLINER/AIRCRAFT is anything these companies fly. Whether a CRJ (Cannadair REGIONAL Jet) or ERJ (Embraer REGIONAL jet), and pretty soon the MRJ (Mitsubishi REGIONAL Jet), they are REGIONAL Jets. And as far as MY understanding, and MY OWN concerns, until any of them are used directly by ANY MAINLINE company to carry passengers on regular basis to points outside this country of more than 3 or 4 hours with enough passengers to REQUIRE at least 3 Flight Attendants, they will remain REGIONAL AIRCRAFTS.
I'm sure some of you by now are thinking the 190s at JetBlue and USair. Good point. They are NOT the main equipment for the routes they SOMETIMES are used for. Those companies use them as relievers (over/under capacity), seasonal (high/low season), and for no longer than 3 hours legs to foreign/outside destination.
Now, please feel free to contradict me with a good argument, preferably with some opposing evidence, and as always in a nice manner.
Thank you and have a great day!.