I hate to tell you this, but most of the folk that lurk in this thread also lurk in the Boiler and PV thread where this has been asked and answered.
I saw this defined somewhere that I was rooting around just this past week. And if I am right, it would have had to have been in a piping code somewhere, since I didn't do any ASME work last week. Sorry, but I can't remember where. It may have been something as silly as wikipedia.
rmw