There is no single way of giving the perfect ultimate defujitio if anything. And there are different approaches to definitions themselves which focus on genesis, function, main attributes or other things. Defining Bitcoin primarily as a ledger means focusing on the underlying technology at its core. If we define Bitcoin as a currency, we focus on its function. Both are essential, and why choose between them if we can combine them? And then, the definition would be something like this: Bitcoin is a blockchain-based currency that can be used for trading, long-term investments and as a medium of exchange.