The way I said it is how every news outlet- Fox, CNN, Reuters, Bloomberg, you name it- and most economists and blogs have used those terms since the Greece crisis began. I think you are not realizing that this is the Euro core or Eurozone core.
What it sounds like is shorthand notation for those parts of Europe that are the least screwed up by Marxist European socialism. With the exception of Germany none of those countries are core countries. They are small countries on the periphery.