No, this isn't a personal question about your belly button. Buyers moving to Northern Virginia often see the phrase "inside" or "outside" the beltway in MLS descriptions and wonder where are these locations on the map. For those who have lived in the Northern Virginia most of our lives we have watched "outside" the beltway on the Virginia side expand far west and south of DC.
Just as the term "slugging" started as a phrase to describe riders using counterfeit tokens, the term inside or outside the beltway started with a different meaning in our area as well. The phrase was originally used politically to describe the fact that most of the power in the DC area was located inside the beltway where the Federal Government is located.
The first written reference to the term is found with the Warren Commission Report about Watergate printed in 1975:
"In the White House of Richard M. Nixon, it was said that Watergate would become serious only if it "got outside the Washington Beltway." Guess we know how that turned out!
Neighborhoods in Arlington, Alexandria, Falls Church and McLean are some examples of neighborhoods inside the beltway in Northern Virginia. Neighborhoods inside the beltway used to lean towards smaller older homes on tree lined lots however there has been a boom of condo developments inside the beltway. With the metro and buses within walking distance, many of the newer neighborhoods quickly gained popularity with buyers wanting to ditch their car for their daily commute.
Outside the beltway has expanded significantly over the past 10 years and some might say that it extends all the way to the Shenandoah Valley where you can find commuter lots for van pools to DC. The selection of neighborhoods outside the beltway is generally more diverse if for no other reason than the space constraints of trying to squeeze homes within a smaller geographic area has been removed. There has been significant new home development outside the beltway in recent years in the western part of Fairfax County, Prince William County and Loudoun County.
So another mystery of local terminology is revealed. If you want to know more about living either inside or outside the beltway give a native of the area a call. I've got a few other localisms I can clue you into as well! 
Cindy, as you know, I'm an Innie, by just about 1 mile.
I've been an innie and now I'm an outie. Still own my innie property though so who knows one day I could be back to my roots :-)
Lol I love your title; though I don't really have any affiliation with North Virgina, though I do have a friend who grew up in Arlington. Would be a fun question to ask them, see if they get it;p
DJ-I'm sure they will. It is one of the first conversation many buyers bring up. They want a house "inside the beltway" so that they aren't sitting on one of the major interstates for their commute to DC.
'Love your title, your topic, your maps... what's not to love about this? I'm sure it's a question that Newbies to the area ask.
Cindy: I've heard that the recovery of the market has started way inside the beltway, like Arlington and Alexandria. Can't wait till it hits Prince William County!
Margaret-It is fun to answer the questions that those of us who live here take for granted after awhile. It reminds me of when I've moved to different areas as a military spouse and had no idea what someone meant.
Chris Ann-some areas inside the beltway haven't even seen a blip. PW and PG are different stories but sales are brisk so at some point the inventory has to be reduced to a managable level.