What is a premiere?

Something called the Amphion Foundation has set up a website devoted to Elliott Carter.  It's standard stuff, though easily navigated, with a canned biography, a list of works and tributes from friends (many of which were printed in the attractively produced booklet handed out at Wednesday's memorial concert with the even more attractively produced program). Most helpful is the schedule of upcoming performances. It's encouraging to see that Mr. Carter's music is surviving him.

I am confused, however, by the notices for Epigrams. The June 22 performance in the UK is labeled the "world premiere," and the August 9 performance at Tanglewood is given as the "US premiere." I could have sworn I heard the piece two nights ago in New York City, which, as I recall, is in both the US and the world. That performance doesn't seem to have counted (and it is not listed as the premiere in the attractively produced program). I'm not up on modern music etiquette, and the only reason I can think of is that attendance at Wednesday's concert was, officially, by invitation only, and the performance was therefore a private occasion. Nevertheless, the public was admitted on a standby basis, and everyone who wanted a seat got one.  I count myself in that category. I was there as the guest of someone whose name appeared on the in-crowd list (thanks, BH), but if I hadn't been, I would have waited outside in the heat and humidity with the rest of the nonentities.

So when is a premiere not a premiere?