Thomas Shey
Legend
Personally, I distinguish between D&D the brand and the genre. Anything set of rules that serve a D&D experience is D&D for practical purposes.
The phrase I use is "D&Doids" or "in the D&D sphere". There's enough similarity within that (as in contrast with all the other games in the hobby) that there are certain common assumptions taken as a given there (and there are plenty) that would not be assumed the moment you get outside of it, and they tend to be clustered together (you'll only rarely find levels in games that don't also do classes and a bunch of the other things, for example, and you rarely see AC as a method of reducing chance to hit in games that don't deal with level-elevating hit points).