what exactly is a package base?

I have been looking at Linux Mint Debian Edition, because for the first time in years I'm interested in distro-hopping; and so I am reading that LMDE uses Debian's package base rather than Ubuntu's--that's the difference. Of course I know what a package is, but it seems that after fifteen years of daily Linux use, I still can't define a package base. My instincts say it's something so obvious that I probably know what it is but can't put it into words--I've never needed to.

Basic research isn't telling me yet--perfectly many explanations of what packages are, but not package bases.