The Base Coin is a coin (an American silver Dollar) belonging to Scrooge McDuck.
The "Base Coin" was the first coin Scrooge McDuck chose to place in his newly-constructed Money Bin. After remaining at the literal foundation of his enormous fortune for over a century, it was charged with large amounts of mystical wealth-generating energy in much the same way as the Number One Dime.
When she learned of its existence following a catastrophe which had quite literally flipped the Bin, revealing the Base Coin, Magica De Spell managed to get Scrooge to trade it in exchange for her help in putting things to right. To Magica's delight, it proved even more powerful a magic-boosting tool than the Dime would have been, but this glee was short-lived; turning the power onto Fethry Duck's mind on a whim, she found that making Fethry conventionally intelligent took so much magical energy that it siphoned the coin clean, rendering it useless, at which point she gave it back to Scrooge.
Behind the scenes
The Base Coin is a major plot point in the 2017 comic story Under Siege!.