I'm not going to define "Dungeons & Dragons" in this entry. That'll have to wait for later, when I have more time to write at length, because it's a complicated question (though not one that defies an answer, despite what the obscurantists will tell you). What I will do, though, is present a small illustration of what D&D was back in 1980 and what it was is something that I both miss and feel is missing from more modern claimants to the name "Dungeons & Dragons."
Here are four pre-generated characters from a module I was re-reading last night, Expedition to the Barrier Peaks. These guys are a nice little window on what D&D once was and what I wish it were again.
Hum Ftr 12, AL N, HP 54, Str 15, Int 14, Wis 12, Dex 13, Con 14, Cha 16, +3 battleaxe, +2 plate mail, +2 shield, ring of fire resistanceThese guys are a snapshot D&D in its most perfect, "real" form, as opposed to an idealized one, which is to say, D&D as it was played at that time, right down to the fighter's having broadly better stats than his companions and more generally useful magic items. This certainly mirrors my own experiences both as a player and as a DM.
Hum MU 11, AL N, HP 27, Str 10, Int 16, Wis 14, Dex 15, Con 14, Cha 14, +2 dagger, gem of seeing, boots of levitation, wand of cold (28 charges)
Hum Cl 10, AL LG, HP 34, Str 12, Int 11, Wis 18, Dex 14, Con 12, Cha 15, +2 mace, staff of striking, ring of protection +3
Human Th 10, AL N, HP 27, Str 10, Int 14, Wis 13, Dex 17, Con 12, Cha 7, +2 sword, bag of holding, cloak of protection +3
Yet, for all that, a 12th-level fighter still only has 54 hit points. Even with his -2 AC, he is nevertheless very vulnerable -- his companions even moreso. Of course "vulnerable" does not mean helpless and, with the gear and class abilities these characters possess, they ought to be able to handle most threats in the module, even the dreaded froghemoth.
I can't yet offer a definitive definition of "D&D," but if you want an illustration of where I'm coming from, I point you to these guys.