I've gone through each rulebook looking for how they use the word "basic," and here's what I found:
Dominion
refers to Copper/Province/etc as 'basic Treasure' and 'basic Victory' cards, available every game
Curse available every game, not identified as 'basic'
broad classification of cards are: basic Treasure cards, basic Victory cards, Trash pile/Curse cards, Kingdom cards (there's an image in the sidebar on one of the pages with cards sorted out like this)
Intrigue
refers to Dominion as the 'basic' game
refers to Copper/Province/etc as 'basic Treasure' and 'basic Victory' cards, available every game
Curse available every game, not identified as 'basic'
Seaside
does not use the word 'basic' at all
Alchemy - first use of capitalized "Basic" and "Basic Supply"
"Dominion: Alchemy is an expansion, and can't be played by itself; to play with it, you need Dominion, or a standalone expansion to Dominion (Dominion: Intrigue). Those provide the Basic cards you need to play (Treasure, Victory, and Curse cards), as well as the full rules for setup and gameplay."
"16 Basic cards, all Potion"
"Potion is a new Basic Treasure card."
"Alchemy includes a Treasure card with rules: Philosopher’s Stone. Philosopher’s Stone is in the Supply if it is selected as one of the 10 Kingdom cards for the game; it is not part of the Basic Supply."
in Potion's FAQ - "This is a basic Treasure card" [only time in rulebook 'basic' is lowercase]
Prosperity
"This is the 4th addition to the game of Dominion. It adds 25 new Kingdom cards to Dominion, plus 2 new Basic cards that let players keep building up past Gold and Province. "
"Dominion: Prosperity is an expansion, and cannot be played by itself; to play with it, you need Dominion, or a standalone expansion to Dominion (e.g., Dominion: Intrigue). Those provide the Basic cards you need to play (Treasure, Victory, and Curse cards), as well as the full rules for setup and gameplay."
"If only Kingdom cards from Prosperity are being used this game, then the Platinum and Colony piles are added to the Basic cards in the Supply for the game. "
"24 Basic cards, 12 each of Colony and Platinum"
Prosperity includes eight Treasure cards with rules on them. They are in the Supply if selected as one of the 10 Kingdom cards for the game; they are not part of the Basic Supply.
Cornucopia
"To play with Cornucopia, you need a complete Dominion game such as the original Dominion or Dominion: Intrigue. These provide the Basic cards you need to play (Treasure, Victory, and Curse cards), as well as the full rules for setup and gameplay."
Hinterlands (pdf seems to be from an image, not a text, so can't copy-paste the text right over)
same thing about needing Dominion or Intrigue for Basic cards
same thing about its Treasure cards not being part of the Basic Supply
Dark Ages
"Dark Ages includes two Treasure cards with rules on them. They are in the Supply if selected as one of the 10 Kingdom cards for the game; they are not part of the Basic Supply."
Dominion has this to say about "the Supply": "Players will not use all of the cards in every game. Other than the players’ starting Decks, the other cards used in a game of Dominion are called the Supply. "
I would define Basic cards as those cards that are part of the "Basic Supply".
Spoils definitely aren't part of the "Basic Supply", while Potion, Colony and Platinum definitely are "Basic." Since Ruins can be bought and gained just like Curses, they are definitely part of the Supply, and I infer that they are part of the Basic Supply; they're not part of the Kingdom cards, and it makes more sense to put them in the Basic Supply then add a third category. I would argue that Shelters are not Basic because they are not part of the Supply.
So the way I see it, all the cards in a Dominion game can be found in one of these places:
-each player's Dominion (Deck); usually starts as 7 Coppers and 3 Estates, sometimes 7 Coppers and 1 Hovel, 1 Necropolis, 1 Overgrown Estate.
-The Supply; when you gain a card or buy a card, it comes from the Supply (unless otherwise specified), which consists of two main parts:
---The Basic Supply (Copper, Silver, Gold, Curse, Estate, Duchy, Province, and sometimes Potion, Platinum, Colony, Ruins)
--The Kingdom cards
-The Trash (and the Trash marker card)
-Other piles/decks; these are not part of the Supply, and are here because one or more Kingdom cards name a special, different location for them (Prize pile, Black Market deck, Spoils pile, Madman pile, Mercenary pile)
tl;dr - michaeljb probably cares too much about definition of "Basic"